Navy Richards-Mason conversions of 18 percussion revolvers by Colt in the 1870s, which I guess is what the Navy must have done instead of adopting the Colt Model 1873 as the Army did. In it, there is some mention about the U. I was reading Adler’s book on metallic cartridge conversions, covering the Post-civil war efforts by Colt to convert C&B revolvers to use metallic cartridges.
