I've done (4) SBRs. They all started life as stripped lowers built as pistols and ran for reliability first. Then, when I get my Stamp, I remove the Pistol Buffer tube, send it for engraving. I use Orion Arms http://www.orion-arms.com/
and they only require that you remove the tube/Buffer/spring to have the front of the Mag. Well. Otherwise, Orion requires thatb you provide a stripped receiver for engraving on any of the other surfaces. I get my Engraved lower back, install a Rifle style Tube and I'm done.