Interesting and excellent pictures. Nice looking rifle.

However, I suspect that your rifle is most likely a model 1868 TD. Yes, strange and contrary to conventional wisdom.

Details on the 1868 TD Springfield:

The Springfield lock plates were recycled from muskets with a redesigned hammer. Lock plates were dated from 1862 thru 1864.

The breech blocks were dated 1868 or 1869, or 1870/Eagle/Arrows/U.S.

Your pictures indicate that both the receiver and the barrel have matching serial numbers which are in the range of production for the model 1868.

Finally, the model of 1870 was the last model that was chambered in 50-70, however that model was not serial numbered.

See The U.S. Springfield Trapdoor Rifle Information Center for additional information and scroll down to the 1868. There are several measurements that you can use to verify what you have.

Hope this helps.