Just FWIW, the term DEWAT (DEactivated WAr Trophy) is pretty specific and there were ATTD instructions for making a DEWAT. The gun did not have to be cut up and most DEWATs can't be told from a live gun from the outside.
The rule was intended to allow vets to deactivate and keep their otherwise illegal war souvenirs; after deactivation, they became legally "non-guns" and could be owned, bought and sold "like table lamps", to quote an ATTD agent.
But then large importers, like Sam Cummings, realized that there was no limit in converting guns to non-guns, so they brought thousands of STEN's, Thompsons, etc., into the country in bond, welded them, and sold them as "non-guns". Those guns, not vet bringbacks, are the bulk of legally owned machineguns today.
After a sensationalist anti-gun "slaughtered in our beds" article in the Saturday Evening Post by Ashley Halsey (later, incredibly, editor of the American Rifleman) the requirement to register DEWATs was added to the GCA '68.
Jim