1955 was an Interesting time period for S&W,
They transitioned away from a standard satin blue finish back to bright blue, it's the end of the 5 screw frame era as the new Combat Magnum is being tooled up to begin shipping after the holiday shutdown in early 1956,
The end of the SW High Speed hammer as soon as the supply is exhausted around 1957 .
Btw the Combat Magnum was the first model to get the new 4 screw frame and also never to have had the high speed hammer,
They were introduced at serial K260,000 in comparison to your Combat Masterpiece Serial K257419