yea what 62 is saying sounds like what I've always read about. Guns were allowed to be replaced with the original SN by the original manufacturer, then when MGs came into the mix the original manufacturers figured why wouldn't this apply MGs as well. I'm in CT and have met a lot of Colt guys over the years who have personally seen Colt MG receivers being replaced with new M-16A2 castings, but you kinda had to know someone there who could hook you up. Ruger would do this as well with their AC556 guns as well, one could send in your beat up MG and they'd replace anything that was bad up to and including the MG receiver with same SN. These guns aren't so much in question as the semi to FA 3rd party guns.
The question arises with the 3rd party conversions, either a home built F1 in Joes garage or a PAWS, Wilson, ect ect ect. if you have a bushmaster converted by PAWS and sent it to bushy to get re-made or even an SP-1 conversion and wanted colt to re-man it to a new lower how can they legally engrave someone else's info on it? PAWS, Joe or Wilson would have to do the work no?