Bri, it may be, the club member provided it FOC and asked me to fit.
There's an alloy boss let into the back of the stock, and a piece of round bar approx. 15mm diameter that is fixed to a flange runs in and out inside this boss. The locking knob is about M5 or M6 and thread is tapped into the side of the boss.
Member also wanted to keep the up and down movement with the Bisley pad, so I made an alloy plate that would carry the Bisley on one side, and the above-mentioned flange on the other. The Bisley backplate is die cast and wasn't man enough to do two jobs. The plate is drilled and tapped to accept countersunk M4 or M5 screws, can't recall which.
You may also see a thin sheet fixed to the back of the stock, this is to hide the mess from a previous attempt by A N Other. When the butt is tight to the butt you can't see anything bout once you extend it all the previous horrors are on show.