Claim it was theirs and was stolen.
Given that Registered Magnums are older guns, chances are many have changed hands at least once, probalby not all throught gun stores. Private transactions have no paperwork required, so how things got from point A to point B can be contested. What was the paperwork required of stores before 1968? Could be a big void out there that can be used to cause legal confusion.
Unless the gun was handed down in the family (good family), an anti-troll or opportunist might do this.
Even if you eventually win, it can be a hassle.
You don't worry about such things? Then give me your name, address, social security number, and mother's maiden name. To save some hassle, what is your driver's license number?