That SN is Ca. 1955. CTG is an abbreviation for Cartridge.
The "Highway Patrolman" is sort of a cut-rate ".357 Magnum" which is an actual model designation used by S&W. The HP, later to become the Model 28, was a less expensively finished version of the .357 Magnum which became the Model 27. The HP was intended as a no-frills working gun principally for law enforcement, and many thousands were used for that purpose. The HP cost about 20% less that the fancier .357 Magnum which was a very important selling point if a police agency of any sort were buying possibly several hundred guns.
Other than that, if it was your dad's gun you probably know more of it's history than any of us ever will.