not to hijack but a quick question concerning SBRs:

If you are building (i.e., buying a lower and buying say a 10.5 upper and slapping them together) what item is going on the paperwork? The lower? Do you have to wait to purchase the upper then once the stamp comes back? I was basically wondering if I could buy the lower now and start the paperwork and once completed buy the upper and slap it together and be good to go, is this correct?