Notwithstanding whether you need to have your NFA SBR engraved or not, I have found a decent machine shop in Pueblo (near the Pueblo Airport) that has a CNC machine easily capable of engraving pretty much anything you want on your AR-15 lower (and probably any other rifle part). In my case I only did lettering, but it was only $15 to have it done.

It took a bit of time to find someone capable and willing of doing the engraving, as your standard trophy shop can't promise that they will match the engraving depth of .003".

O K Tooling
(719) 948-3950
31670 Ozark, Pueblo, CO 81001

It will save time if you bring a lower vice block in with your lower to make it easier to mount in the CNC vice gear.