Colt, like all the other manufacturers in the US, was prevented from making cartridge revolvers by the Rollin white patent, which finally expired in 1869. This left them limited to their percussion revolvers, the 1849, 1851, 1860, and 1862 models in particular. These were phenomenally popular guns, but quickly becoming obsolete in the face of the new cartridge technology. Colt would introduce the Peacemaker in 1873, but in that brief gap between the patent expiration and their new purpose-built cartridge revolver they needed something to put on the market.
The answer was a series of cartridge conversions - guns which could fire the new cartridges but could be built from the large existing stockpiles of percussion revolver components. The first such gun was the Thuer conversion, followed by the much better Richards conversion, and finally the simplified Richards-Mason conversion (in addition to the purpose-built Open Top).
These conversion were offered form the Colt factory on all the major models of percussion revolver, with the small pocket guns the most popular. Because Colt had a large supply of existing parts and could sell these guns cheaper than their other new designs, the conversions would remain available and selling through the 1870s.
The Richards conversion is distinctive for having a barrel-mounted rear sight, as well as a remarkably modern floating firing pin.