It looks to me that the .410 started out as a 2" to be shot in 45/70s.
Not with a folded crimp, but just a round card sealed with wax.
That would have been in 47/70 brass to start.
Now I see others speculating the same sort of thing:
Then, over time, the .410 body got skinnier and longer, and the rim got thinner and smaller in diameter.
CAUTION: The following post includes loading data beyond currently published maximums for this cartridge. USE AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL assume any liability for any damage or injury resulting from use of this information.
I have gone to 85 kpsi with 45/70 in a handi rifle and 45 Colt in a Steven .410 break action. That is the pressure where the brass [without extractor groove] starts to yield.
The Guide gun is not built as stout and those guns, and so by the time the brass gives up the ghost, the rifle might already be damaged. So working up a load until the brass changes is not a good idea.
The .410 is registered at 12.5 kpsi.
The 1885 is good for ~ 40 kpsi in the 450 Marlin.
What I would do is try to find some 2.5" 410 data, and use the same wad and charge for the 2" 45/70.
I would use a large pistol primer. That thin primer cup would be the canary in the coal mine. It will look horrible if the pressure gets to high.
I would seal the mouth of the case with a round card and wax from a candle.
The difference between a 2.5" shot shell with a crimp that unfolds and a 2" shot shell without one is small.
The pressure should be somewhere around 12.5 kpsi then.
If it is a little over it will not matter, as the 1885 has plenty of safety margin for 12.5 kpsi loads.