I just sold a Colt flattop upper receiver that had "COLT 223" rollmarked on the LH side but no forge code or proof marks on the RH side. It was in original dark gray and a "small pin" too.
I have been told that mine was made by Colt but it was a conversion type upper assy'. In other words suppose you had a complete rifle with an A2 upper, BUT you also wanted a flattop. Well, with this assy' you swap your original upper for this one and, VOILA, instant flattop on the same lower!
Sounds like you haxe a conversion upper assy" too but with a forge mark. Go to BIGGERHAMMER.net and see if they have your forge code.