Finally picked one up. Has this, in my guess is a switch on side. Not sure what it's for?

works possibly like a PRV, pressure relief valve .. ...

Randy the weird thing is it has electrical contacts or screws when I took covers off. Hard to see in picture.

Looks like a Light switch

There is a light installed on unit but not through this gadget. There are 3 connections on this third is out of sight. When I do a search the result finds some but not all of these units have this.

What does the switch have to do with when using over 90 psi for trucks??