[Shooter 2.5]

I didn't have the picture on my 'puter so I thought you had an original.

45-70's came out in 1873 so I thought you might have had a 50-70 or a rebarreled rifle.

[M.K. Borders]

There were several model years for the 45-70 "Trapdoor", the first of course being 1873.

Trapdoors were being produced from surplus musket stock as early as 1865 and chambered with much larger loads.
Though there were several 45-70 models fielded, they all tended to look like the the 1873. The differences in these models were things like barrel bands, sights, bayonets etc.

I would have to look it up, but I believe the last 45-70 goverment model to be fielded by the Army was 1885. These weapons were in use to well past 1900.