Okay I just re-reread your post and you say .25-20
SS That's a different cartridge than the Win .25-20. Cartridges of the World lists factory loads with 86 gr bullets at 1410 fps. Bertram sells brass for it, and RCBS dies will be a special order proposition.
It's still true that the 44 is a cast action, so hot-rodding is discouraged, unlike the forged action 44 & 1/2 model.
If it's actually a .25-20 Winchester, then Hodgdon and other sources should give mild load data for it.
Neat find, in any case. The workmanship on the Stevens of that era was very nice. Sort of an American equivalent to a Rook rifle. Load it up, and shoot something tasty.
