Fantastic! Great to have another Z9 owner on here. A few of your questions I can answer off the top of my head, and a few I'll need some more time to answer..
First, when I got the machine, there was a small copper tube attached to the right side terminal on the back of the pressure gauge. Nothing was attached to the left-hand terminal (or the right-hand one if you are looking at the back of the machine). What, if anything, is supposed to be on the other terminal? No diagram shows that terminal.
Mine is the same way - nothing on the right-hand side as looking at it from behind. One of my sets of diagrams confirms that nothing goes in there. I've just assumed that it is the same gauge used for another model that does utilize that port.
Where does the wire from the sight glass indicator go?
A photo would be good, but if that is the level probe, then mine connects to my GICAR auto-fill, so I assume that yours goes to your RALS box. Depending on how much you plan on using your Z9, it had been suggested to me several times that I do away with the auto-fill and just manually fill as needed (turns out that my GICAR is fine, and I did figure out the wiring, so I kept it).
Finally, there is the old RALS box "per autolivella caldaia," for automatic levelling of the boiler. That has pins that fit into holes in a piece located at the back left side of the frame, or, in the photo, straight back along the frame from the Sirai switch. Why are there both the pressure switch and the auto levelling switch (I realize that both were originally on the machine)?
The pressure switch turns on the heating element, and the auto-fill brings in water (if I understand your question correctly).
If Phillip has not yet gotten his gas tubes etc. straightened out, I think I can help with that.
That would be great - I'm messing around with my Elektra T1 and SAMA Lusso, so I put the Z9 on the back burner - again, and again.
Welcome to HB, Robert!