Our latest Pacific Marine Fire Engine

[Gruber Bros.]

And finally, on this Pacific Marine Engine and a couple of others we have noticed this device located just above the passenger step & below the cab (visible on pic 1). What does it do?

It looks like something to do with air. The dust cap is removed at the top to reveal a thread, and I think the oily bit at the bottom is a connector of some sought. It has a gauge on the right, but on this one the glass is warped and opaque and I haven't been able to get it out yet. All I can read is "Made in England", and the left end of the scale starts with "15". Has anyone seen it before, and if it is some sought of air regulator, what would be driven with it?

Thanks, Ben.