Converting the Webley Senior from a piston ring to a leather seal?

[eyebull]

Agree with that 100%.
The FWB 300 rifle uses a split ring as well, though I am sure steel ones with a stepped split section (higher performing) are advertised as non original replacements.

How does a stepped split section work? I can't quite visualise it.

I think there is absolutely nothing that a half decent split ring cannot handle in airgun cylinder pressures since they have been used for ages in high compression turbo-charged diesel engines and last many years in the harshest compression cycle one can imagine.

I think a reasonably competent person with a lathe can turn a few rings effortlessly if the cylinder bore is still true and the piston groove/cylinder dimensions are known. I would hate to loose more of what little power these fine pistols make by fitting leather washers.

Wandering off topic now but I wonder how feasible it would be to convert, say, a HW95 to a PB ring seal?