Technically if the lower receiver wasn't marked as a pistol or receiver on the 4473 you and the dealer filled out, than it can still be considered an SBR.
That's what I thought too... but according to everything I've found, including Mr. Celedonio from the Harrisburg office of the BATFE, seems to refute that.
According to him, the lower receiver does not need to have any special engraving on it for building a pistol. It is already serial numbered and that number is on the FFL's books. It doesn't matter if you build a pistol or a rifle at that point.
You do not need to have a letter of certification from the manufacturer stating that it is for a pistol because of the generic nature of AR lowers. The only time you have to engrave the lower is if you are making a SBR.
He goes on to say that the BATFE realizes that people like to change their AR's into different configurations and there is not a problem with that as long as they meet all the regs.
This would be different, obviously, if I purchased a complete AR that was recorded as a rifle on the books, then wanted to put a short upper and a pistol buffer on it... but when you buy a stripped lower, it can be used for whatever you want... at least this is what I'm getting from all the other people who have researched this.