It seems nearly impossible to determine what would be an incomplete receiver, and what would be a block of metal. There would need to be a whole set of regulations or a law defining exactly how far along the part becomes an "incomplete receiver".
That seems like a critical issue, since even a licensed firearms manufacturer could end up shipping or having un-serialized incomplete receivers in existence during the manufacturing process. What happens if a firearms manufacturer decides to use an external CNC machine shop to do some work on their receivers before they finish the machining in house. Do they need to serialize the partially finished receivers?
TOTALLY correct regarding manufacturing and part serialization and outside seservices.... Big headache.
Its crazy. Not well thought out seems to me.