I bought a stripped AR lower 3 or 4 years ago. When the ban ended last Sept. I decided to build a pistol. I called the state police and asked them what I needed to do. They (there is a woman who answers all firearms questions, but I can't remember her name) told me that they do not consider a stripped lower a pistol until it is assembled. Therefore, they suggested assembling it into a pistol, obtaining permit to purchase from the local PD, and then taking it in for registration.
I went to my local PD and obtained a purchase permit and filled it out. I also told them what I was doing. I then brought the AR pistol back to them for safety inspection/registration. Mine had a 11.5" bbl and a telestock buffer tube on it but I took it in with the upper and lower separated (just in case they had an issue). It confused the desk officer a bit, but I explained it to him. The woman in the records bureau could not find the name of the manufacturer (it was a Century marked CMT lower) on her pistol list so she had to add it, but it was no problem.
Since then I also built and registered a Stag lower as a pistol using the same 11.5" upper.
Here's something to consider. If the bbl on your registered AR pistol is less than 16" you can't put a stock on it because that makes it a short barreled rifle. If 16" or more you can put a stock on it
and use it as a rifle.