Some interesting disinformation floating around this thread.
The only thing that makes a lower a "pistol" lower is for it to have been assembled as a pistol. You can buy a lower advertised and marked as a pistol lower complete with extension and brace. If the first thing you do is put a stock and rifle upper on it, before completing a pistol assembly, it is now and forever more a rifle lower. You can not legally make a pistol using this lower.
If you buy a rifle lower complete with extension and stock and the first thing you do is put a brace and pistol upper on it, it is a pistol lower. You can make this lower into a rifle and back to a pistol by changing out the rifle and pistol parts.
This is all true regardless of any markings on the lower, the advertising, what a dealer incorrectly put on the 4473,... The first configuration of the weapon, rifle vs. pistol, determines what the lower is in the eyes of the ATF.