Index.of.finances.xls.39

DataHoarder_99

Found Index.of.finances.xls.39 in old backup—what is this format? Index.of.finances.xls.39

If I rename to .xls or .xlsx , Excel just crashes. Hex dump shows 39 00 00 00 at offset 0x1A, which might be a version marker. DataHoarder_99 Index