Quote Originally Posted by WebleyWombler View Post
Washer is 1 piece , looks like blue PTFE. holding screw is much smaller diameter that the one holding a leather washer to a Mk 1. Piston is about 1/2" longer than a Mk1.

Fit a Mk1 piston into it and the pistol won't cock, even with a long link senior barrel and a junior spring fitted.
That confirms it then - an unstamped B as you originally mentioned. It's the first unstamped B I've heard of though.

John M