Is the device you are asking about not a pressure regulator to control the amount of tension on the belts?
I don't "think" so as this device is on the unloading line of the cylinder. Pressure regulation I would think would have been on the supply end to the cylinder.
From the foggy recesses of my brain this AM...and a schematic... No air pressure on the belts, the system won't allow the alternator to be loaded, and the low air light will be on. Somewhat goof proof, eh? On my '75 MC8, the "Gen Control and Low Air switch" is plumbed into the loop to the alternator belt tensioner. So, this is where the air pressure is sampled for the low air warning, as well as allowing the alternator to come online once sufficient belt tension (air pressure) is achieved to prevent throwing them off or slipping under load. Good engineering? Two birds, one stone.
I think I'll look at this intermediate device.
I can't believe there aren't others out there with the same configuration.
Somewhat striking out here.