diff --git a/java/hiddenapi_modular.go b/java/hiddenapi_modular.go
index 44cdfa5..534a814 100644
--- a/java/hiddenapi_modular.go
+++ b/java/hiddenapi_modular.go
@@ -295,6 +295,12 @@
 	return dexJar.Path()
 }
 
+// HIDDENAPI_STUB_FLAGS_IMPL_FLAGS is the set of flags that identify implementation only signatures,
+// i.e. those signatures that are not part of any API (including the hidden API).
+var HIDDENAPI_STUB_FLAGS_IMPL_FLAGS = []string{}
+
+var HIDDENAPI_FLAGS_CSV_IMPL_FLAGS = []string{"blocked"}
+
 // buildRuleToGenerateHiddenAPIStubFlagsFile creates a rule to create a hidden API stub flags file.
 //
 // The rule is initialized but not built so that the caller can modify it and select an appropriate
@@ -345,7 +351,8 @@
 	// If there are stub flag files that have been generated by fragments on which this depends then
 	// use them to validate the stub flag file generated by the rules created by this method.
 	if len(stubFlagSubsets) > 0 {
-		validFile := buildRuleValidateOverlappingCsvFiles(ctx, name, desc, outputPath, stubFlagSubsets)
+		validFile := buildRuleValidateOverlappingCsvFiles(ctx, name, desc, outputPath, stubFlagSubsets,
+			HIDDENAPI_STUB_FLAGS_IMPL_FLAGS)
 
 		// Add the file that indicates that the file generated by this is valid.
 		//
@@ -904,7 +911,8 @@
 	// If there are flag files that have been generated by fragments on which this depends then use
 	// them to validate the flag file generated by the rules created by this method.
 	if len(flagSubsets) > 0 {
-		validFile := buildRuleValidateOverlappingCsvFiles(ctx, name, desc, outputPath, flagSubsets)
+		validFile := buildRuleValidateOverlappingCsvFiles(ctx, name, desc, outputPath, flagSubsets,
+			HIDDENAPI_FLAGS_CSV_IMPL_FLAGS)
 
 		// Add the file that indicates that the file generated by this is valid.
 		//
@@ -968,13 +976,29 @@
 	return patternsFile
 }
 
-// buildRuleRemoveBlockedFlag creates a rule that will remove entries from the input path which
-// only have blocked flags. It will not remove entries that have blocked as well as other flags,
-// e.g. blocked,core-platform-api.
-func buildRuleRemoveBlockedFlag(ctx android.BuilderContext, name string, desc string, inputPath android.Path, filteredPath android.WritablePath) {
+// buildRuleRemoveSignaturesWithImplementationFlags creates a rule that will remove signatures from
+// the input flags file which have only the implementation flags, i.e. are not part of an API.
+//
+// The implementationFlags specifies the set of default flags that identifies the signature of a
+// private, implementation only, member. Signatures that match those flags are removed from the
+// flags as they are implementation only.
+//
+// This is used to remove implementation only signatures from the signature files that are persisted
+// in the sdk snapshot as the sdk snapshots should not include implementation details. The
+// signatures generated by this method will be compared by the buildRuleValidateOverlappingCsvFiles
+// method which treats any missing signatures as if they were implementation only signatures.
+func buildRuleRemoveSignaturesWithImplementationFlags(ctx android.BuilderContext,
+	name string, desc string, inputPath android.Path, filteredPath android.WritablePath,
+	implementationFlags []string) {
+
 	rule := android.NewRuleBuilder(pctx, ctx)
+	implementationFlagPattern := ""
+	for _, implementationFlag := range implementationFlags {
+		implementationFlagPattern = implementationFlagPattern + "," + implementationFlag
+	}
 	rule.Command().
-		Text(`grep -vE "^[^,]+,blocked$"`).Input(inputPath).Text(">").Output(filteredPath).
+		Text(`grep -vE "^[^,]+` + implementationFlagPattern + `$"`).Input(inputPath).
+		Text(">").Output(filteredPath).
 		// Grep's exit code depends on whether it finds anything. It is 0 (build success) when it finds
 		// something and 1 (build failure) when it does not and 2 (when it encounters an error).
 		// However, while it is unlikely it is not an error if this does not find any matches. The
@@ -986,7 +1010,14 @@
 
 // buildRuleValidateOverlappingCsvFiles checks that the modular CSV files, i.e. the files generated
 // by the individual bootclasspath_fragment modules are subsets of the monolithic CSV file.
-func buildRuleValidateOverlappingCsvFiles(ctx android.BuilderContext, name string, desc string, monolithicFilePath android.WritablePath, csvSubsets SignatureCsvSubsets) android.WritablePath {
+//
+// The implementationFlags specifies the set of default flags that identifies the signature of a
+// private, implementation only, member. A signature which is present in a monolithic flags subset
+// defined by SignatureCsvSubset but which is not present in the flags file from the corresponding
+// module is assumed to be an implementation only member and so must have these flags.
+func buildRuleValidateOverlappingCsvFiles(ctx android.BuilderContext, name string, desc string,
+	monolithicFilePath android.WritablePath, csvSubsets SignatureCsvSubsets,
+	implementationFlags []string) android.WritablePath {
 	// The file which is used to record that the flags file is valid.
 	validFile := pathForValidation(ctx, monolithicFilePath)
 
@@ -1003,6 +1034,10 @@
 			Implicit(subset.CsvFile).Implicit(subset.SignaturePatternsFile)
 	}
 
+	for _, implementationFlag := range implementationFlags {
+		command.FlagWithArg("--implementation-flag ", implementationFlag)
+	}
+
 	// If validation passes then update the file that records that.
 	command.Text("&& touch").Output(validFile)
 	rule.Build(name+"Validation", desc+" validation")
@@ -1076,12 +1111,16 @@
 	// Generate the filtered-stub-flags.csv file which contains the filtered stub flags that will be
 	// compared against the monolithic stub flags.
 	filteredStubFlagsCSV := android.PathForModuleOut(ctx, hiddenApiSubDir, "filtered-stub-flags.csv")
-	buildRuleRemoveBlockedFlag(ctx, "modularHiddenApiFilteredStubFlags", "modular hiddenapi filtered stub flags", stubFlagsCSV, filteredStubFlagsCSV)
+	buildRuleRemoveSignaturesWithImplementationFlags(ctx, "modularHiddenApiFilteredStubFlags",
+		"modular hiddenapi filtered stub flags", stubFlagsCSV, filteredStubFlagsCSV,
+		HIDDENAPI_STUB_FLAGS_IMPL_FLAGS)
 
 	// Generate the filtered-flags.csv file which contains the filtered flags that will be compared
 	// against the monolithic flags.
 	filteredFlagsCSV := android.PathForModuleOut(ctx, hiddenApiSubDir, "filtered-flags.csv")
-	buildRuleRemoveBlockedFlag(ctx, "modularHiddenApiFilteredFlags", "modular hiddenapi filtered flags", allFlagsCSV, filteredFlagsCSV)
+	buildRuleRemoveSignaturesWithImplementationFlags(ctx, "modularHiddenApiFilteredFlags",
+		"modular hiddenapi filtered flags", allFlagsCSV, filteredFlagsCSV,
+		HIDDENAPI_FLAGS_CSV_IMPL_FLAGS)
 
 	// Store the paths in the info for use by other modules and sdk snapshot generation.
 	output := HiddenAPIOutput{
diff --git a/scripts/hiddenapi/verify_overlaps.py b/scripts/hiddenapi/verify_overlaps.py
index 940532b..f985a49 100755
--- a/scripts/hiddenapi/verify_overlaps.py
+++ b/scripts/hiddenapi/verify_overlaps.py
@@ -107,7 +107,8 @@
         return read_signature_csv_from_stream_as_dict(f)
 
 
-def compare_signature_flags(monolithic_flags_dict, modular_flags_dict):
+def compare_signature_flags(monolithic_flags_dict, modular_flags_dict,
+                            implementation_flags):
     """Compare the signature flags between the two dicts.
 
     :param monolithic_flags_dict: the dict containing the subset of the
@@ -130,7 +131,7 @@
             modular_row = modular_flags_dict.get(signature, {})
             modular_flags = modular_row.get(None, [])
         else:
-            modular_flags = ["blocked"]
+            modular_flags = implementation_flags
         if monolithic_flags != modular_flags:
             mismatching_signatures.append(
                 (signature, modular_flags, monolithic_flags))
@@ -140,7 +141,13 @@
 def main(argv):
     args_parser = argparse.ArgumentParser(
         description="Verify that sets of hidden API flags are each a subset of "
-        "the monolithic flag file.")
+        "the monolithic flag file. For each module this uses the provided "
+        "signature patterns to select a subset of the monolithic flags and "
+        "then it compares that subset against the filtered flags provided by "
+        "the module. If the module's filtered flags does not contain flags for "
+        "a signature then it is assumed to have been filtered out because it "
+        "was not part of an API and so is assumed to have the implementation "
+        "flags.")
     args_parser.add_argument(
         "--monolithic-flags", help="The monolithic flag file")
     args_parser.add_argument(
@@ -149,18 +156,30 @@
         help="A colon separated pair of paths. The first is a path to a "
         "filtered set of flags, and the second is a path to a set of "
         "signature patterns that identify the set of classes belonging to "
-        "a single bootclasspath_fragment module, ")
+        "a single bootclasspath_fragment module. Specify once for each module "
+        "that needs to be checked.")
+    args_parser.add_argument(
+        "--implementation-flag",
+        action="append",
+        help="A flag in the set of flags that identifies a signature which is "
+        "not part of an API, i.e. is the signature of a private implementation "
+        "member. Specify as many times as necessary to define the "
+        "implementation flag set. If this is not specified then the "
+        "implementation flag set is empty.")
     args = args_parser.parse_args(argv[1:])
 
     # Read in all the flags into the trie
     monolithic_flags_path = args.monolithic_flags
     monolithic_trie = read_flag_trie_from_file(monolithic_flags_path)
 
+    implementation_flags = args.implementation_flag or []
+
     # For each subset specified on the command line, create dicts for the flags
     # provided by the subset and the corresponding flags from the complete set
     # of flags and compare them.
     failed = False
-    for modular_pair in args.module_flags:
+    module_pairs = args.module_flags or []
+    for modular_pair in module_pairs:
         parts = modular_pair.split(":")
         modular_flags_path = parts[0]
         modular_patterns_path = parts[1]
@@ -170,7 +189,8 @@
             extract_subset_from_monolithic_flags_as_dict_from_file(
                 monolithic_trie, modular_patterns_path)
         mismatching_signatures = compare_signature_flags(
-            monolithic_flags_subset_dict, modular_flags_dict)
+            monolithic_flags_subset_dict, modular_flags_dict,
+            implementation_flags)
         if mismatching_signatures:
             failed = True
             print("ERROR: Hidden API flags are inconsistent:")
diff --git a/scripts/hiddenapi/verify_overlaps_test.py b/scripts/hiddenapi/verify_overlaps_test.py
index ead8a4e..0a489ee 100755
--- a/scripts/hiddenapi/verify_overlaps_test.py
+++ b/scripts/hiddenapi/verify_overlaps_test.py
@@ -221,7 +221,8 @@
         modular = self.read_signature_csv_from_string_as_dict("""
 Ljava/lang/Object;->hashCode()I,public-api,system-api,test-api
 """)
-        mismatches = vo.compare_signature_flags(monolithic, modular)
+        mismatches = vo.compare_signature_flags(monolithic, modular,
+                                                ["blocked"])
         expected = []
         self.assertEqual(expected, mismatches)
 
@@ -232,7 +233,8 @@
         modular = self.read_signature_csv_from_string_as_dict("""
 Ljava/lang/Object;->toString()Ljava/lang/String;,public-api,system-api,test-api
 """)
-        mismatches = vo.compare_signature_flags(monolithic, modular)
+        mismatches = vo.compare_signature_flags(monolithic, modular,
+                                                ["blocked"])
         expected = [
             (
                 "Ljava/lang/Object;->toString()Ljava/lang/String;",
@@ -249,7 +251,8 @@
         modular = self.read_signature_csv_from_string_as_dict("""
 Ljava/lang/Object;->toString()Ljava/lang/String;,public-api,system-api,test-api
 """)
-        mismatches = vo.compare_signature_flags(monolithic, modular)
+        mismatches = vo.compare_signature_flags(monolithic, modular,
+                                                ["blocked"])
         expected = [
             (
                 "Ljava/lang/Object;->toString()Ljava/lang/String;",
@@ -266,7 +269,8 @@
         modular = self.read_signature_csv_from_string_as_dict("""
 Ljava/lang/Object;->toString()Ljava/lang/String;,blocked
 """)
-        mismatches = vo.compare_signature_flags(monolithic, modular)
+        mismatches = vo.compare_signature_flags(monolithic, modular,
+                                                ["blocked"])
         expected = [
             (
                 "Ljava/lang/Object;->toString()Ljava/lang/String;",
@@ -281,7 +285,8 @@
         modular = self.read_signature_csv_from_string_as_dict("""
 Ljava/lang/Object;->toString()Ljava/lang/String;,public-api,system-api,test-api
 """)
-        mismatches = vo.compare_signature_flags(monolithic, modular)
+        mismatches = vo.compare_signature_flags(monolithic, modular,
+                                                ["blocked"])
         expected = [
             (
                 "Ljava/lang/Object;->toString()Ljava/lang/String;",
@@ -296,7 +301,8 @@
 Ljava/lang/Object;->hashCode()I,public-api,system-api,test-api
 """)
         modular = {}
-        mismatches = vo.compare_signature_flags(monolithic, modular)
+        mismatches = vo.compare_signature_flags(monolithic, modular,
+                                                ["blocked"])
         expected = [
             (
                 "Ljava/lang/Object;->hashCode()I",
@@ -311,7 +317,47 @@
 Ljava/lang/Object;->hashCode()I,blocked
 """)
         modular = {}
-        mismatches = vo.compare_signature_flags(monolithic, modular)
+        mismatches = vo.compare_signature_flags(monolithic, modular,
+                                                ["blocked"])
+        expected = []
+        self.assertEqual(expected, mismatches)
+
+    def test_match_treat_missing_from_modular_as_empty(self):
+        monolithic = self.read_signature_csv_from_string_as_dict("")
+        modular = self.read_signature_csv_from_string_as_dict("""
+Ljava/lang/Object;->toString()Ljava/lang/String;,public-api,system-api,test-api
+""")
+        mismatches = vo.compare_signature_flags(monolithic, modular, [])
+        expected = [
+            (
+                "Ljava/lang/Object;->toString()Ljava/lang/String;",
+                ["public-api", "system-api", "test-api"],
+                [],
+            ),
+        ]
+        self.assertEqual(expected, mismatches)
+
+    def test_mismatch_treat_missing_from_modular_as_empty(self):
+        monolithic = self.read_signature_csv_from_string_as_dict("""
+Ljava/lang/Object;->hashCode()I,public-api,system-api,test-api
+""")
+        modular = {}
+        mismatches = vo.compare_signature_flags(monolithic, modular, [])
+        expected = [
+            (
+                "Ljava/lang/Object;->hashCode()I",
+                [],
+                ["public-api", "system-api", "test-api"],
+            ),
+        ]
+        self.assertEqual(expected, mismatches)
+
+    def test_empty_missing_from_modular(self):
+        monolithic = self.read_signature_csv_from_string_as_dict("""
+Ljava/lang/Object;->hashCode()I
+""")
+        modular = {}
+        mismatches = vo.compare_signature_flags(monolithic, modular, [])
         expected = []
         self.assertEqual(expected, mismatches)