Bushmaster –– B.F.I. –– did not manufacture their own lower receivers prior to May 19, 1986. Instead, they bought finished existing Title 1 receivers (mostly Sendra receivers from Drasen Corp), milled-and-drilled them and filed Form 2's, and registered them as MGs. They had both Sendra (or whatever) and B.F.I. markings.

Yes, I know there are at least seven snake-marked "transferable" Bushy M16's out there. I would not touch one with the proverbial ten-foot pole.

Prior to about 1999, you were required to sibmit a Form 5 to transfer your NFA item to any repair facility, including the original manufacturer; a second Form 5 had to be submitted and approved before the manufacturer could ship it back to you. So BATFE has records of all of the transferables which were ever returned to the manufacturer (many even plainly state "receiver replacement" as the reason).

The Big 3 firearms makers that would on request replace receivers of transferable MGs are Olympic, Colt and Bushmaster. So they simply cross-reference for those.

Receiver replacement happened a lot in the late '90s after passage of the so-called Assault Weapons Law. Because that law simply banned manufacture of AWs after a certain date, existing preban guns were not affected, and BATFE allowed replacement of damaged or faulty parts including receivers.

A lot of owners thought this meant they could also legally replace MG receivers, but the way the two laws were written was not the same. FOPA '86 –– which included the MG ban –– covered all machine guns, and made them all illegal for civilian ownership. But proof that the MG had been made and registered prior to May 19, 1986 was an affirmative defense against prosecution. Since the registered part of the MG is the receiver, that piece of metal must have existed as a registered MG prior to the '86 date. In the law's (and BATFE's) view, a replacement receiver –– even with the same serial number –– manufactured after that date is a post-'86 MG and cannot be owned by civilians.

When BATFE found out about Olympic's receiver replacement program, they shut it down. Colt never openly acknowledged that it would replace receivers, though there are obvious examples out there (the receiver forgings have features not available until after '86). Bushy didn't do as many replacements, but as we all know, they are out there.

Right now, BATFE is rounding up all the MACs-reborn-as-other-MGs that were built and sold a coupla years ago. Just because you have a valid Form 4 for an MG does not mean ATF can't come knocking on your door and confiscate it, with no compensation. It has been happening for years, and will keep happening as long as BATFE has its current mission (restricting our right to own and bear firearms).

The original pre-1986 Bushys would be marked either Sendra or Essential Arms (and possibly some others). In addition, Bushmaster as the 07/02 SOT performing the conversion engraved them with "B.F.I.", standing for Bushmaster Firearms Industries. They did not engrave "Bushmaster" on these receivers, although that engraving could legally be added afterward, as long as the "B.F.I." remained as well.