Now I remember, because TheFinn and I had this gauge mentioned before in another list of mine. Do you remember too, The Finn?
I think I was wondering about the readings and after you said that this gauge would show hydraulic pressure of some kind (which would make sense because of the placement on the panel) I just posted the excerpt from the Captain Sim manuals and the issue was "solved" (meaning that we weren't speaking of it anymore).
So, I follow you on the thinking that Captain Sim named this thing "engine start air pressure indicator" but it actually is the "rudder hydraulic pressure gauge".
All we need would be a scan of your book concerning this gauge.

I'd fix the list then with this picture.

I can report the same as Mark, engine start without external air is impossible. This would be another proof for the gauge showing something else but start air pressure.
Again, the placement near the other hydraulics stuff suggests that TheFinn is absolutely right and that just the manuals are wrong on this.

I have to check the frequency knobs too, thanks for the HU on them.