I thought the Combat Masterpieces only came in a 4" barrel?
That's not a Combat Masterpiece pictured. It's a Target Masterpiece.
A little S&W K-Frame back ground. The K frames started in 1899 with Military and Police Hand Ejector model, aka the M&P. Prior to WWII S&W made a target version of the M&P with adjustable rear sight, and longer bbl. After WWII they offered the M&P as the K-38, K-32, and K-22 Target Masterpiece. They had ajustable rear sights, 6" or 8 & 3/8" heavy bbls, and a partridge type front sight.
After numerous request and special orders from the FBI, LAPD, and others, S&W offered the K-38 Combat Masterpiece, with the same rear sight, a 4" bbl, and a Baughman ramp style front sight (quicker draw from a holster) as a standard production revolver. The K-22 Combat Masterpice was sold as a training companion. I do not know of a K-32 in Combat configuration.
In 1957 S&W switched from names to numbers. K-38 Masterpieces became the 14 (Target) and 15 (Combat). The K-22 Masterpieces became the 17 (Target) and 18 (Combat). I can't remember the year, but not too long after '57 S&W discontinued the 18, and offered the 17 in either Target or Combat configuration. There are known examples of special ordered 14s in Combat configuration and 15s in Target configuration as well. The bbl. length and front sight type are the distinguishing features.