You need to go to the ATF webiste, and read their FAQs. As I read them, do it once, no. Do it several times, yes. for example, buy a shipment of surplus Mausers. Take off wooden stocks, install new synthetics, drill and tap for scopes, mount scopes, offer for sale- manufacturer.
HOWEVER, customer brings you surplus Mauser,wants stock replaced, drilled and tapped, scope mounted- NOT a manufacturer- that is a gunsmith.
See why I said read the FAQs?
What we have here is... failure- to communicate.