Originally Posted By: 3DHUSKERAh... yes they can. Like I said in my post above, some do and can be marked that way.
Here is one example:
http://www.andersonrifles.com/product/ar15-pistol-lower-receivers/
Technically, markings are irrelevant for the ATF's rulings. The ATF doesn't care that it's stamped "pistol" on the side - stripped receivers sell as "Other," regardless of whether it's a pistol receiver or rifle receiver, or an ambiguous AR receiver (1911 frames sell as "other" been there, done that).
Additionally, it's NOT illegal to convert a pistol lower into a rifle lower, it just cannot be converted back (arguably).
I suppose one could argue (with the ATF) that a lower that's marked "pistol" was purchased with the intent to be a pistol, so when they built it into a pistol, then rebuilt a rifle, they'd at least be able to ARGUE (again, with the ATF) that it's still a pistol lower - kinda like the on-going issues that the T/C Contender had with riding both sides of the fence, or the guys that tried to rebuild XR-100 rifles into XP100's. With an AR lower, they'll likely lose. Once it has a stock on it, it's a rifle, and can't ever be a handgun again.
So, again, it depends if this guy bought an assembled lower, or a lower kit. A stripped lower in a kit is still a stripped lower, and if the selling FFL marked it as they were supposed to as "other" on the 4473 AND he never assembled it as a rifle, then it could get built into a pistol. (Although if you didn't own a rifle, and had an AR pistol and an AR rifle stock, you technically own an SBR).
If it was an assembled lower, it can never become a pistol.