I think you're probably right on target with that. I never got much into all the intracacies of the Colt models and designations, and I don't have any of my issues of SAR handy to check, but I think you're spot on. I know I've never seen a factory marked XM177, but I wasn't thinking it may be designated by Colt otherwise like a RO609 or RO610.
Johnny - you're also correct that most MP5's are converted HK94's - either by a registered conversion part like a sear, or by drilling the front pivot pin hole in the receiver. To be completely truthful, my "M16s" are really only semi auto AR15's converted by registered sears and links. I still refer to them as M16's, M4's, etc, even though it would be more proper to call them full auto Bushmaster XM15's. I'm sure some purists would probably like to string me up for my willful misuse of the nomenclature, but I'm really not a purist, and its so much easier to simply refer to as an M16, M4, etc. since its a functional equivalent thereof. I still refer to my sear converted, full-auto SAR8 as a "G3", even though it was made in Greece and not as factory G3 or even as a machinegun.