Warren Foster (who was retired GM exec) told me that GM marketed a sort of "crate engine" that was based off of the 54 261. They had the later production dates described. The theory was that they wanted to offer a "drop in" replacement for the earlier 216s when they wore out so that all the accessories would not have to be changed. The serial number pads were blank when they left the crate so the original numbers could be stamped in. My bet is that this is what you have.
