As sort of a generic answer, immediately after the CW, the US government had on hand an enormous quantity of muzzle loading rifles that were becoming obsolete, as breechloaders came into use. Consider also, that enormous debt had been run up by the gov't to finance the war.
So, a low cost solution to equip the post war army with breech loading rifles was needed.
Many muzzleloading muskets were converted to breechloaders by cutting away the breech of the ML barrel and fitting a trap door mechanism, similar to the later Trap Door Springfield. The musket barrels were relined to .50 caliber, and the cartridge was the 50-70.
I believe this conversion of ML's took place starting in 1866, or thereabouts.
Shortly after that, Springfield armory began making new Trap Door Springfields, rather than converting ML's. I believe that process began about 1868. Originally, the new manufactured rifles were 50-70's, but before long all the trap doors in service were refitted for the 45-70 cartridge.
