Okay, I'm bad. You're right if a "receiver" (a lower) that has never had a stock mounted on it, is bought, and documented as such, then you can build it either into a rifle or pistol. But if you assemble it as a rifle first you cannot make it into a pistol later. But if you make it into a pistol first then you can go either way later.
I mistakenly thought the determination was made by the manufacturer. But that's only so if they assemble it into a rile. If they sell it virgin then the buyer gets to decide.
BUT THE QUESTION REMAINS. How do NY and Feinstein's proposed laws treat a virgin lower?
Are they assuming it is automatically an AR? What if, when "they" come knocking I don't have an upper. What if I only have a bolt action or cross bow upper? What if I have assembled it as a bolt action but have an AR upper in the back of the safe? Does their definition of AWB include pistols or just rifles? Is this a loop hole?