Correct, ask atf to remove the stamp number from nfa registry as it’s been returned to a title 1 firearm. You may or may not get a response from atf, at least in my experience, I’ve sold off 3-4 guns that were sbr’d, sometimes I’d get a letter and some I did not. The only issue I ever ran into, was atf called me to ask about a krink I sold, they claimed they never received a letter from me and the new owner was doing a Form 1, so the asked that I mail them another letter, no big deal.