The ATF is not known for independent thinking...or thinking at all, to be frank.

As far I as I know, shipping a receiver (considered to be a gun by the AFT) or even a complete gun to the manf or gunsmith for work does not require an FFL. It is NOT a transfer.

However, I have had UPS (another bastion of intelligence) insist on having a copy of the receiving gunsmith's FFL in order to ship via them. This happened to me once only.

Cheers

Stephen