Phil,
I know nothing about these guns, but the only reason I know why a rifle might have a silver disc in the stock is to be engraved or personalized... with the owner's initials, for example. To my way of thinking, if the original owner did not ever get it engraved, it makes it tougher to identify the original owner, but gives you a perfect opportunity to have your OWN initials engraved! (Once you are certain you will never part with it ;-) )