I certainly don’t see any issue as is. It obviously depends on how much value you place on the letter. They are after all firearms and not some multi million dollar piece of art. As the above poster states, if the letter doesn’t match.....

Really not that much different than buying a used vehicle and finding the window sticker folded in glove compartment. If the sticker indicates a blue car and the one your sitting in is painted pink, it may have been altered.