Prior to 1956, K and N (large) frame revolvers had a 5-screw frame. There were four screws in the side plate (the upper side plate screw was larger than the other three) and a cylinder stop plunger screw in front of the trigger guard. On September 7, 1955, S&W authorized the deletion of the upper side plate screw for K-frame revolvers (a year later for N-frame revolvers). The Combat Magnum (later the Model 19) was put into production in late 1955 and was the first model to be made with the 4-screw frame (i.e. three screws in the side plate and the cylinder stop plunger screw in front of the trigger guard). On November 28, 1961, S&W improved the cylinder stop that resulted in the deletion of the cylinder stop plunger screw. By 1962, revolvers with a 3-screw frame (i.e. three screws in the side plate) were being shipped.

Bill