Hi willib
so how long is the piece of string?
I can't see what type of compressor you have there, but a few things to take into account on a simple system.
It must have an inlet, signified usually by some form of air filter... or turn the pump and feel for the suction side.
It must have an output (the other one)
Once we have that we can have any number of gadgets hanging off the side. If the compressor has a reciever (holding tank of some type) it will have a copper pipe to connect it to the compressor output, and this may be the one your looking at.
It will have a sample of the output going to a shutoff switch to control the on/off switching. It will probably have some of the outlet directed to a pressure gauge, and more of the output will be paralled with an o/pressure valve. (generally on the reciever).
Then you may have a tap, a water trap, a pressure converter to stabilise the output over a wide range of tank pressures .... as you can see you can keep on adding stuff so it's hard to guess at what you have, and the physical structure can vary as much as the imagination, all hoses connecting everything, or manifolds where no pipes are needed and it gets done like the controller in an auto transmission.... like a three dimensional circuit board.
...........so.... a picture..... (and a small one please.) I have not followed this thread well as I havent downloaded the large pics you have.
Well this may not have helped yet, but maybe soon......
.........oztules