The original serial number has obviously been scrubbed....and by the patina over the scrubbed area I'd say some time ago at that. For what reason is unknown, but stolen property sort of comes to the top of the list. Now comes the even more interesting issue. Unless the owner can prove the scrubbing occurred outside of the country prior to import or prior to 1968, the rifle is exhibit "A" of a felony charge of altering or obliterating a serial number. Owning such a firearm without said proof is tantamount to being the original perpetrator. If it was mine I'd quietly get a replacement receiver and rebuild the rifle around it......and destroy the altered receiver. Now the odds of the authorities becoming aware of this rifle is somewhat small if it stays where it is and doesn't go through a public sale. However ultimately it will come to light if not addressed and the holder of the rifle at that point will likely be the one left to account for it.