I think if you supply more information, someone may help/ 1860s had 1 piece cylinders. For most conversions, the rear was milled off, and a breech ring installed. The only two piece conversion cylinders back then that I know of, were much like current R&D conversion cylinders, where the back came off to load the cylinder. Remington did a lot of two piece conversions. I also read (I think in McDowell's book) that Springfield Armory tried some 5 shot, two piece (R&D style) conversion cylinders for the 1860 that were for .45 Colt.