What JamesK says. Title still rests with the original owner and it must be surrendered to the original owner. A good example is the art stolen by the Nazis that have been sold by Nazi era art dealers to collectors and museums. Even thought said stolen painting has been out of the owner/family's hands for decades, it's still theirs.
Would suggest a FOIA search under ATF to see if they can find it.
Vigilantibus et non dormientibus jura subveniunt. Molon Labe!