sentences:  The first metallic cartridge revolver made by Colt was the Thuer-Conversion Model Revolver, a design that would not require a cylinder with cylindrical chambers so as not to infringe on the Rollin White patent.A small number (about 1000–1500) of Model 1851 Navy revolvers was converted, using front-loaded, slightly tapered cartridges to fit the chambers of the cylinder reamed to a slight taper.After the expiration of the Rollin White patent (April 3, 1869), Colt 1851 (and 1861 Navy) Revolvers were converted or newly made to fire .38 rimfire or centerfire cartridges, the "Colt Model 1851 Richards- Mason Conversion" by the Colt factory.