diff --git a/scripts/backfill_sidecars.py b/scripts/backfill_sidecars.py
index b71bd89..2b9533e 100644
--- a/scripts/backfill_sidecars.py
+++ b/scripts/backfill_sidecars.py
@@ -54,14 +54,26 @@ log = logging.getLogger("backfill_sidecars")
 
 
 def _looks_like_event_file(path: Path) -> bool:
-    """Same heuristic as the importer CLI."""
+    """Same heuristic as the importer CLI.
+
+    Filters to BW (Series III) event files only — Thor (Series IV)
+    `.IDFW` / `.IDFH` files share the store but have their own ingest
+    path (`WaveformStore.save_imported_idf`) and are NOT decodable by
+    `event_file_io.read_blastware_file`.  Their sidecars are populated
+    at ingest from the paired `.IDFW.txt` ASCII report; nothing the
+    backfill regenerates would improve on them, so we exclude them
+    from scope.
+    """
     if not path.is_file():
         return False
-    if path.name.endswith((".a5.pkl", ".sfm.json")):
+    if path.name.endswith((".a5.pkl", ".sfm.json", ".h5")):
         return False
     ext = path.suffix.lstrip(".")
     if not (3 <= len(ext) <= 4):
         return False
+    # Thor IDF files share the .{W,H}-suffix shape but aren't BW.
+    if ext.upper() in ("IDFW", "IDFH"):
+        return False
     if not (ext[-1].upper() in {"W", "H"} or ext.endswith("0")):
         return False
     try: