How do they work? does anyone have a cross section drawing or something?

ratworks do a nice one for the hw pcp airguns but sadly cant get them for my old webley, rws etc even though they use the same style gauge!




how does one charge the gun with a filler probe but have the gauge right at the very end of the cylinder? i would have a try and making one but cant understand how that would look/function internally - hoping someone can point me in the right direction or lend some ideas.


Thanks all