The over-pressure valve has a heavier metal gauge coil and shorter length spring as it is not expected to open except in very unusual circumstances. The pumping valves work every time the gun is being loaded with the spear, hence have lighter springs. Are you sure that nothing is missing as right now I don't see how that gun goes together to trap the rear end of the pumping barrel. Plus there are threaded plugs on each face of the bulkhead when it would seem logical that they were only in the rear face abutting the nose of the grip handle. I will try drawing it all up in a sketch and see if I can figure out what is missing. The last diagram with two gun profiles was a start, now I have to finish it. I am thinking of diagrammatically showing the gun assembled without the tank from the side on or profile view.