Picked up an old Smith & Wesson revolver today and am curious of the model and year. There isn't a model number inside the swing arm like newer models but there is the number 56740 on the arm and the frame. The serial number on the butt is 721769. I'd like to change the cheap plastic grips but need to know the frame designation too. Most likely a K but not sure. The logo is on the "driver's" side and only says MADE IN USA stamped on the frame below the cylinder. The barrel only has 721769 stamped on it on the bottom side. A 38 special round fits perfect. Any insight would be helpful. Thanks!