In 1937, Colt introduced this improved version of the Ace Pistol. It utilizes a floating chamber, invented by David "Carbine" Williams the firearm's designer who invented the "Short Stroke Gas Piston", that is the basis of the MI carbine, while serving time on a Southern chain gang. Colt advertised that this pistol, with its floating chamber would give the Service Model Ace the reliability and "feel" of a .45 Auto. Today, owners of Service Model ACE pistols find they require regular maintenance and cleaning in order to keep the close-fitting floating chamber from binding. Furthermore, fouling appears to be much worse with some brands and types of ammunition. Most owners feel that although the perceived recoil of Service Model ACE is noticeably greater than that of ACE, it falls far short of a .45 Auto's recoil. Serial number is prefixed by letters "SM". External configuration is the same as Ace. Slide is marked "Colt Service Model Ace .22 Long Rifle". Most were sold to the Army and some on a commercial basis. There were a total of 13,803 manufactured before production ceased in 1945. NOTE: Add 20- to 30 percent for both models for finish 99-100 percent; 35 percent for "US Property" marking on blued pistols.