In 1871, Colt's introduced its first revolver models using rear-loaded metallic cartridges: the .41 caliber Colt House Revolver (also known as the Cloverleaf for its four-round cylinder configuration) and the .22 cal Colt Open Top Pocket Model Revolver. However, Colt's wanted a more powerful practical handgun loaded with metallic cartridges so the company put forward William Mason, who in 1871 began work on Colt's first .44 caliber metallic-cartridge revolver: the Colt Model 1871-72 Open Top. The company registered two patents for the Open Top, one in 1871, the other in 1872, the same patents mentioned in the markings of Colt Single Action Army revolvers, a nowadays legendary and long produced model, improved and based on the Open Top. Production of the Open Top started in 1872 and stopped in 1873 when the Single Action Army model started to be delivered to the US Army. However, the Open Top was already a completely new design. The parts, for example, 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 rimfire 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 topstrap, similar to the Remington revolvers, and placed the rear sight on the rear of the frame; he consulted with Richards on some other improvements. The first prototype of the new gun was still chambered in .44 rimfire, but the first model was in the newest caliber known as the .45 Colt.