I was told that GM offered a sort of "crate motor" that was a 261 in the 54 water pump style that may show a later casting date. They had nothing stamped in the distributor pads. Warren Foster (wdoftexas) told me they were marketed as a direct replacement when the old 216s in the bigger trucks "expired" so they didn't have to change accessories to whatever. I think they were even painted yellow. May be one of those
