some of my ithaca 66 has factory checkering and some do not all mine have ss# most start with 66 but not all of them it is located on the left side if the receiver. still locking for a barrel for you. the guy that had the 12 gauge slug barrel put it back up as a bid now not buy it now. at $10.49 now
http://m.ebay.com/itm/231168152801?nav=SEARCH