In 1871, Colt employee Charles Richards was awarded a patent for converting Colt percussion models to breech loading cartridge revolvers. The Richards cartridge conversion was an instant success. On July 2, 1872, William Mason, another Colt employee, was awarded a patent for an improvement to the Richards model. As percussion parts ran short, the 1860 Richards-Mason barrel was changed from the profile of the earlier percussion model, to the sleek profile found on the 1871-72 Open Top. Conversion models remained popular with cowboys (many originals will be found with imprints of fence staples on the butt) even after the introduction of the 1873 Colt®. This was due to the low cost of conversion models.