To prove that it's not restricted, get someone who knows shit about firearms.
No other way around it. Ask to get a supervisor to inspect it or if you're friends with the local CFO, LE, or CF get them to check it out and tell them if it's a receiver or not.
It might help to bring a picture of an actual receiver and a picture of a mechbox to compare and contrast.
AFAIK, optics aren't restricted/prohibited (well maybe the eotech ones and ones that fall under the US exportation ban list).
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE