Ruger pistol model MK I, caliber 22, with integrated suppressor.
It was very accurate pistol, very light and extremely reliable.
Contrary to most weapons that can be 'silenced', the caliber 22 is an already-subsonic caliber, so in order to get the maximum 'discretion' you don't need to use special ammunitions.
It was a very popular weapon between spies, guns for hire and CIA agents.
It was mostly used by the 'shadow men' (reconnaissance scouts) of the two Baker teams
Some very special units use it nowadays still ( they use in fact the Mark II, a slightly updated version).