And this is why the ATF rules are so whacked. The same thing is legal in one config and not in the other.And when I say "started out as a pistol lower" I mean the first time it was built it was built as a pistol. That is what determines whether it is a pistol or rifle. If first completed as a pistol it will always be a pistol lower and can be swapped back and forth as often as possible. If first completed as a rifle it can only ever be a rifle. It really makes no difference what an FFL transferred it as. Unless the manufacturer registered it as a rifle lower when they serialized it, you can build it either way. An FFL does not have the authority to change the classification of a weapon unless they are also the manufacturer and they actually assigned the serial number.
ZERO common sense applied. Course, I feel the same about silencers and SBR's.... ZERO thought involved in making arbitrary rules.