When the ATF assigns a new serial number the prefix is always ATF. the Listed serial number " ATF135XX-XX " fits the style and format of a ATF reassigned serial number. If it is on the grip instead of the butt, the butt may have been damaged or defaced. The ATF would have a record of it, S&W wouldn't have a clue about the new number. Also remember, ATF didn't engrave the new number, either the police department did ( which I doubt ) or a gunsmith who requested the renumber.