” Possession of an unregistered firearm ( U.S.C. Section 5861(d)). Probably 80% receivers that were completed and did not have serial numbers.”
Must have been set up for full auto, else not illegal.
“unregistered” law only applies to NFA weapons AFAIK.
It is illegal to possess “firearms” that do not have a serial number, if you are manufacturing them for sale.
In the documents linked, U.S.C Section 5861(d), one of the provisions is for the requirement of serial numbers.