I’m kind of new to PCP rifles but I have never seen one like this. I’m not sure if it’s missing something but I can’t find a fill port or gauge or anyway to fill this thing. This is what the tube looks like.
You are very likely looking for information on "John Bowkett" stalker Rifles from the UK. (your trigger guard certainly looks like his work. Oh and More pictures please). Had one of two old BSA Spitfire's he did, OUTSTANDING! Pretty well the first to make .308's & such (some silenced) and ship to the U.S. . A rare rifle so I am happy to take that bothersome rig off your hands, trae you and airforce or DAQ. Kidding -but- friend.
The screw head underneath the stock, the screw removed. Not sure if I need a special adaptor or something. There is no gauge anywhere. I saw something online about a huma reg that might screw into the end of the tube. Is that advised or…?