I have an 01 and I called ATF and asked if it were legal for me to put a COMPLETE lower onto a COMPLETE upper and SELL the gun as a complete rifle, mind you, this entire process entails a whopping TWO pins.
The answer: NO, that's is manufacturing!!
Its legal for individuals to buy uppers and lowers and then sell their completed gun, but dealers ( holding an 01 ) cannot, go figure....