All: The word over on 24hourcampfire.com is that the "YG" is an assembly or inspection number, not a model number, and that what I've got is your basic Model 99 EG. Rob, the YG is not the same size as the serial number where stamped on the butt plate, but is on the butt itself. Thanks for steering me over there.
I also removed the forend and found a 1950-range serial number, a couple of thousand lower than the number found elsewhere on the gun. Don't know how uncommon that is, or just how bad it is. It certainly looks like it could have left the factory that way, and it doesn't look replaced or "wrong." Thoughts on that? Thanks again. TT