Receivers are more "fluid" and more easily identifed as "other", but stand alone frames are "other" as well. IE., slap my frame on my mechtech CCU and it's a rifle lower.That's what I was thinking. This is not an AR lower receiver, it is a pistol frame.
That does not, however, mean all FFL's understand that and may only want to list them as "pistol". IOW, they should know better but I wouldn't take it for granted and would have a dialogue with them before filling out any paperwork.