I don't know much about the originals either.

Looks to me to be an earlier model than 1874, by the humped lock plate.

Is that 1869 I see stamped on the barrel?

If the date is 1859, then it appears to be a converted percussion Sporter Sharps. The conversions kept the humped lock plate from the "Maynard Tape Primer" mechanism.

It could be the welding noted on the hammer, was due to a later Model 74 top end being welded to the cap-lock Sharp's "1859" LOWER. Just when this was done, could be an interesting aspect of this rifle as well. Most simply had the hammer nose modified to hit a firing pin instead of a percussion cap or priming disk.

The answers to these questions will be interesting.

Is it chambered/barreled by any chanced, for the .44-77 - or .50/70?