If you purchase a stripped lower, it can be used for a rifle or a pistol build. A 1911 lower frame carries the serial number, and it, too needs to be transferred through an FFL. Whichever part of the firearm that has a serial number is the portion that needs the FFL transfer. A Ruger semi auto .22 pistol, for instance has the serial number on the upper half, So the upper half of that firearm is the portion that needs the FFL transfer.
No, what I’m saying is that if you fab one up from either scratch or an 80% build or whatever.
I have a 1911 80% blank that has no numbers on it. As I understand it, and many people have done this, when I mill it out and finish it it doesn’t need a number.
So I figure the same concept applies to the AR pistol. If you get an 80% lower, finish it, and then slap on some pistol spec parts I would think that it wouldn’t need a number either.