Mason left Remington Arms in 1866 to work for Colt as the Superintendent of the armory. Along with Charles Richards, Mason patented designs to convert percussion revolvers into rear-loading metallic cartridge revolvers. Those converted revolvers are identified as the "Richards-Mason conversion". After working on these conversions, Mason began work on Colt's first metallic cartridge revolvers in 1871: the Colt Model 1871-72 "Open Top" revolver was the third such pistol, following the .41 caliber House Pistol and the .22 caliber seven-shot Open Top. The Open Top .44 was a completely new design and the parts would not interchange with the older percussion pistols. Mason moved the rear sight to the rear of the barrel as opposed to the hammer or the breechblock of the earlier efforts. The caliber was .44 Henry and it was submitted to the US Army for testing in 1872. The Army rejected the pistol and asked for a more powerful caliber with a stronger frame. Mason redesigned the frame to incorporate a top strap, similar to the Remington revolvers and placed the rear sight on the rear of the frame. The first prototype was chambered in .44 rimfire, but the first model was in the newest caliber known as the .45 Colt.