Several years ago a lady came to me with a pair of guns she had found in her uncles house after he had passed. I bought them because one was an original Richards-Mason 1851 Navy conversion. What I didn't know was that it was a "OMC" as I was told by a friend on the Colt forum. OMC referred to Original metallic cartridge. Colt had a bunch of 1851 parts left before the switch to the single action army, and made up just over 3000 from scratch. It is in 38 RF and has a period holster made from a Civil War holster. One of my prized possessions.