I’ve fitted one to a B2. The key thing is the piston must be a centre rod latching type, as the HW’s are. Secondly the positional relationship between the sear and trigger needs to be reasonably close so the trigger locates in the original slot in the stock.

I’ve no idea what work is needed to modify a Webley mk III, but the B2 needed a completely new back block making. I do have a Webley Falcon I also toyed with idea of modifying. The original back block possibly could be butchered, but for the amount of work I’d prefer to make a new one.

Cheers
Greg