56-50 Spencer
The 56-50 Spencer Rimfire was used in the
Spencer Model 1865 carbine, but it was designed by the
Springfield Armory in late 1860. Evidence suggest that
these specimens  are C. D. Leet  & Company, Springfield,
Ma. from 1860 to 1864 production. They have 3 bullet
crimps on the case sides and 2 tool marks on the base on
the cartridge.