I do not have one of these pistols and have never seen one in the flesh or a picture of one apart-----------but at a guess and from the previous posts, I would guess it is like a sidelever Tell 2,Acvoke, Hyscore sort of thing, having the barrel running through the centre of the piston? On the Tell 2 I made a piston washer and had to re pean over the boss on the piston after fitting. If you don't want to disturb the boss, it is possible if you make up a piston washer and soak it in water that you can fit it over the boss and then try to form the washer by squeezing it with your fingers or a jubilee clip and then get it into a piece of tube of a similar diameter to the cylinder. I actually managed to do this with both a Harrington Gat and a Milbro G10 that had a knackered washers--and it worked.
I don't like to comment on things I don't really know about but as you are not having much luck, maybe a look at the piston and see if the above are options?
If you get a better answer, I'll delete this.
Leather is a very good material for some of these oldies for seals.