Lots of strange things were allowed and disallowed over the years. Registered (factory, not drop-in) AK and M16 sears were married to receivers post 86. Stens were transformed into Sterlings. There were changes needed if the original manufacturer didn't do the conversion, however original manufacturers were allowed to transform them into true Sterlings. Charlie Erb was one of them doing it up until the mid 2000's. Oh and I have an M16A1 I picked up in 1998 that left Colt in 1995 to John Keosayian then to Dennis Todd, then to a non-sot, then to me. I have a Colt letter from when they were still doing them for machineguns. What ruined it for everyone was the DBs taking MAC serial numbers and welding a cut out of the number to all kinds of things such as M60's, 1919's and GE mini-guns. Gemtech had a righteous bitch as AAC was essentially passing off their cans with Gemtech markings. Kinda like putting ford VIN on a Chevy I guess. Manufacturers found clever ways around some things like the Vollmer 21's where they folded the receiver up and welded the serial numbered portion above the feed mechanism. This was allowed because the original number was not altered and never left the receiver. Strange times.