The receiver if it's a stripped and not an 80% finished job must be marked with the Company that made it, City and ST of the company that made it, a serial number and a model number. These marks are REQUIRED by the ATF. It's usually on the left side of the receiver on the mag well. Same side as the Bolt Release. For instance, Spike's Tactical are all ST15. Bushmasters are XR15.
Oh, and CONGRATS!