It may be a simple question, but I guess nobody who knows the answer has seen it yet.
I live in a state where SBRs and SBSs are illegal. I only know the laws regarding them as far as what I can't do; for example, I know how to avoid getting in trouble for constructive possession of an SBR when buying parts for an AR pistol or a suppressor pin-and-weld job.
I think you need to mark the receiver with your name (or trust's name) and the city/state (I have no clue what the minimum size of the lettering is). But I don't know for sure because it's illegal in my state and therefore I've never done it. Sorry.
0331: "Accuracy by volume."