it's also on the underside of the empty shell ejector, facing the back of the cylinder. Also the grips may have a # too. My "S" M&P has one grip stamped with another #, and my # in pencil
Some really nice guns. I lucked out and got a barely used 1948 M&P 4", s/n S 978xxx, shipped 3/1/48 to Honolulu Sporting Goods. I'm told it's a late transition model. Had to get a "letter from Roy" too!