IThis pistol started life as a Smith and Wesson model 39 , modified by the addition of a slide lock and suppressor, this effectively converts it into a single shot weapon but makes it very quiet in use .
The same weapon was used by US navy seals with more elaborate modifications , These modifications included a slightly longer barrel threaded at the muzzle to accept the suppressor, high-profile sights that could be seen and used with the suppressor, and the grip was reconfigured to accept originally a Browning P35 double-column, 13-round magazine—later to utilize a Smith & Wesson designed 14-rounder.
A slide-lock switch was added to the frame to prevent the slide from operating, thus making the pistol a single shot. This was done to prevent the enemy from hearing the pistol operate. The suppressor was so effective with sub-sonic ammunition that the loudest part of the gun was the cycling of the slide. A detachable wire shoulder stock and holster were also part of the weapon package.
The navy seals used the weapons for more than 20 years , The CIA service record is unknown. This particular weapon was given to Captain Peter Mason by James Angleton , head of the CIA counter intelligence section.