That worked a treat, the HW70 is a VERY simple pistol thankfully. The strip & re-lube has seen the power double from 1 ft/lb to 2

Just enough for cans at 6 meters.
Looks like the only way to get a Weihrauch piston seal is to buy the later type piston

The only other way is to machine off the seal mounting thingy & drill & tap it to take one of the 25mm leather seal conversion kits, but it'd need a lathe for that, I wouldn't trust myself to be central enough with a drill.
To say the spring has masses of preload & is a bugger to get in, it's very easy to cock

A bit of lathing in that area could allow the fitment of a chunkier Welsh Willy spring. That & a new seal could be a nice improvement.
Blimey, it's July already, & only 17 sleeps to Fish & Chips in The Anchor
.