The Trane Model "E" compressor loads/unloads using hydraulic (oil) pressure. The hand hole cover has a plate with it that directs the oil to the various cylinders as needed. The covers can be for electric activation, pneumatic, or plane old suction activation. There is a screw in the cover under a big cap and a spring loaded bellows inside the hand hole to allow for adjustment. The spring can be changed for various pressure ranges (2 choices as I recall). There are many of these units still pumping away and rebuilt ones are available. I doubt there are any new ones left out there since Trane stopped making the Big Iron compressors years ago. But you can rebuild them yourself. You haven't lived unless you have replaced a 100 hp motor in one of these things.