diff --git a/packages/populace-build/tests/test_us_fiscal_refresh_builder.py b/packages/populace-build/tests/test_us_fiscal_refresh_builder.py
index 7a81adda..59f52c11 100644
--- a/packages/populace-build/tests/test_us_fiscal_refresh_builder.py
+++ b/packages/populace-build/tests/test_us_fiscal_refresh_builder.py
@@ -1264,6 +1264,42 @@ def test_unsupported_ledger_filter_metadata_all_value_is_noop() -> None:
     }
 
 
+def test_identity_ledger_filter_qualifiers_are_inert_not_unsupported() -> None:
+    """Series-identity qualifiers pass the guard; unknown domain filters fail.
+
+    Build M's sparse run stopped here: the #405 NIPA and LIHEAP targets carry
+    fact metadata identifying WHICH published series the registry selected
+    (a NIPA table line code, the LIHEAP state-programs count) — applied at
+    fact-selection, restricting nothing in the microdata. The guard now
+    recognizes the reviewed identity-qualifier class as inert while any
+    unknown ledger_filter_* key stays fatal, so a genuine domain filter can
+    never be silently ignored.
+    """
+
+    builder = _load_builder_module()
+    specs = (
+        SimpleNamespace(
+            name="bea_nipa.cy2024.total_wages_salaries.a034rc.wages_salaries_amount",
+            metadata={"ledger_filter_bea_nipa.series_code": "a034rc"},
+        ),
+        SimpleNamespace(
+            name="hhs_acf_liheap.fy2024.national_profile.state_programs.households_served",
+            metadata={
+                "ledger_filter_administering_entity": "state_programs",
+                "ledger_filter_program": "liheap",
+            },
+        ),
+        SimpleNamespace(
+            name="unknown_domain_filter",
+            metadata={"ledger_filter_novel_dimension": "specific_slice"},
+        ),
+    )
+
+    assert builder._unsupported_ledger_filter_metadata(specs) == {
+        "unknown_domain_filter": ("ledger_filter_novel_dimension",)
+    }
+
+
 def test_eitc_child_count_mask_supports_soi_child_groups() -> None:
     builder = _load_builder_module()
     counts = np.asarray([0, 1, 2, 3, 4], dtype=np.float64)
diff --git a/tools/build_us_fiscal_refresh_release.py b/tools/build_us_fiscal_refresh_release.py
index 28dbfd0c..09f207d8 100644
--- a/tools/build_us_fiscal_refresh_release.py
+++ b/tools/build_us_fiscal_refresh_release.py
@@ -505,6 +505,22 @@ def _automatic_gc_suspended():
     }
 )
 
+#: Series-identity qualifiers, distinct from the domain filters above: each
+#: identifies WHICH published series the registry selected (a NIPA table line,
+#: a LIHEAP program count), was applied at registry fact-selection time, and
+#: restricts nothing in the microdata — there is no household-level "series
+#: code" to filter on. The materializer must treat them as inert provenance;
+#: listing a key here asserts a reviewer verified it is identity-only. Unknown
+#: ledger_filter_* keys remain fatal so a genuine domain filter can never be
+#: silently ignored (the Build M sparse stop that motivated this class).
+IDENTITY_LEDGER_FILTER_METADATA_KEYS = frozenset(
+    {
+        "ledger_filter_bea_nipa.series_code",
+        "ledger_filter_administering_entity",
+        "ledger_filter_program",
+    }
+)
+
 FISCAL_TARGET_SOURCE_KEYS = {
     "cbo": "Congressional Budget Office revenue projections",
     "cms_aca": "CMS ACA marketplace enrollment public use files",
@@ -3441,6 +3457,7 @@ def _unsupported_ledger_filter_metadata(
                 for key, value in metadata.items()
                 if str(key).startswith("ledger_filter")
                 and str(key) not in SUPPORTED_LEDGER_FILTER_METADATA_KEYS
+                and str(key) not in IDENTITY_LEDGER_FILTER_METADATA_KEYS
                 and not _is_noop_ledger_filter_value(str(value))
             )
         )