I had thought originally that this rifle was an 1861 Springfield converted to a trapdoor and the bore lined to fire 50/70, but now it seems these 1868 rifles were made originally as 50/70 trapdoors and used leftover parts from 1861 rifles. The lockplate is dated 1863, the breech 1869. I assume the same tooling and parts were used as from the 1861 rifles, the stocks, locks and other parts look the same.
It's a nice rifle (from the pics) I got it for a little less than other 50/70 Springfields I have seen, probably due to an old repair to a crack through the wrist (which concerns me a bit for shooting) and it seems like the metal was lightly cleaned. I'm still pretty enthused to add the 50/70 Springfield to my collection, it's my goal to get at least one example of each US military infantry rifle from the very early flintlocks up to the present day (well, a Class III M16 and original flintlock)