First, congratulations on having your grandfather's gun. I know personally what a wonderful thing that is.
At K261118, it is a near certainty that your revolver dates to 1956 and it will have a four screw frame. While the SCSW is technically correct when it refers to 1955, only a small number were actually assembled in December of that year (3, IIRC). Full production began in 1956.
While we cannot see it in your photos, I'm sure there is a fourth screw in the front of the trigger guard. Also, there were no five screw Combat Magnum revolvers. This was the first model on which the fifth screw was eliminated.