The way ATF amnesty worked in the past for retroactive stuff, like the USAS-12 shotgun (now DD), was you filled out a Form 1 and listed the manufacturer on the receiver (just like any other F1 for an existing firearm you bought new) and you didn't have to engrave because it left the factory/importer in the same configuration you have now, so you haven't "made" anything. ATF waived the $200 tax from the time of their ruling declaring it a DD (in the 1990s) to like 2002ish.
Even today if you have an unregistered USAS-12 receiver (disassembled with the barrel far away, of course) you can still F1 it and register it as a DD, but you pay the $200 tax and you're assumed to be the maker (since you're going to put it together from parts, right?) so you're required to engrave it.