An M&P built sometime between 1914 and 1920; grips look correct for the timeframe. Assume you mean there is no SN on the grip panel, hopefully there is one on the butt of the frame. The SN on the grip would have been in pencil at that time, I think, and might still be there but very hard to see. Sometimes looking at the grip on an angle in bright light, sunlight or flashlight, and sometimes with magnification the pencil lead numbers can still be seen. I was looking for the SN on a 2nd Model .455 MKII the other day that I had noted had no SN on the grip but checked again in bright light and could just barely make out three of the numbers in the five digit SN. It seems to be in pretty good shape although I can't tell if it has been refinished although it may well have been. Value isn't high in the market but since it came from your Grandpa it is priceless. Wish I had one like it from my Grandpa. And welcome to the Forum, hope we see you here often.

Jeff
SWCA #1457