Had a quick look at home...your gun sounds like a Imp Mod D from the dims you give.....if the patent on the underlever is numbered 8761/04, then this all ties up.........the link I have is 6 and 3/16" from the centre of the pivot hole to the front edge of the piston engagement lug.........PM me if you want to test fit it, as I am Worcester sat a.m.