Someone put that front sight on, and a GI would probably not have had access to the equipment to do that. So maybe a combination of a "duffel cut" and some other work later on.
FWIW, the only "registration" would have been the permission paper, and whether the GI had that or not was often irrelevant. Army regulations required that all the GI's belongings had to fit in his duffel bag; unit commanders could choose whether or not to enforce that, and many didn't. But some did, so if a GI was in such a unit, his souvenir had to fit in the bag, whether he had a paper or not. As to the paper, again sometimes MP's at the point of debarkation would ask for it, sometimes there was no check at all.
The father of a friend brought back a P.35(p) Radom through NY; the MPs took the gun, and a welder in NYPD uniform welded up the muzzle and the gun was given back. Obviously, that was not a universal practice, but it was done to comply with NYC law, which even then was completely insane on handguns.
Jim