I don’t know if there’s a formal letter on them, but I think they treat them as MGs on their own the same as they do for a DIAS. The “selector switch” alone is possession of an unregistered post-86 MG.
This is correct. When dealing with machine guns you have to determine which part is the “machine gun”. For an AR conversion using standard M16 parts it is not the M16 parts (both sears, trigger, safety, hammer) that you can just buy anywhere but the lower receiver once you drill the third whole. For my MP5 clones it is the full auto trigger pack that I can buy anywhere once I milled it to fit in a semi auto receiver. For the AR where you don’t modify the receiver it is the lightning link or DIAS. In all cases it has to be “marked” and is the registered component.
I have a “80%” glock selector that is engraved and ready for me to finish manufacturing and register (form 2). That is the machine gun. It does not matter if I have a glock to put it in, all by itself it is the machine gun.
I will reserve comment on what I think of the NFA, the ATF interpretation of it, etc.
Sometimes it is pure judgment on the part of the 07/SOT. I know some that marked the mp5 receiver. Odd choice since it is I modified but not wrong. I know someone who purchased a metal “lower” for a scar and marked that and out in full auto parts. It’s like marking your SBR on the receiver or barrel. You can do both but if you do the receiver you can put any barrel you want on the receiver. The atf requires you mark something and has rules on what is allowed. There is usually a “good” choice and some bad ones.