Is a "J" frame revolver chambered for the .22 MRF cartridge, believe it was offered with a 3.5" barrel in both Nickel & Blue and even a choice of round or square butt. I'm thinking the round butt is less often found as well as the nickel finish. No longer in production, the M-651 (Stainless Version) replaced it and I think its now out of production. The M-51 is the 1960 .22/32 Kit Gun Magnum Rimfire. Produced 1960-1974. Serial number between 52637-135465. Cherish this revolver since it came from your Dad and is also a very desirable S&W Revolver in its own right! One other thing, back in the day the factory would fit a .22 LR cylinder to the M-51 which would make it capable of firing either with the proper cylinder installed.