So wait a min, are you saying you got a preliminary ok but not full approval?https://www.atf.gov/rules-and-regulations/docs/undefined/faqfinalrule2021r-08f-correctedpdf/download
I found this, this might clear up some questions. Basically TLR; you get a free Form 1 and the ATF considers your brace a stock. Once said form is back approved, you can put whatever the hell you want on the back of your new SBR.