Unloader Valve as others have stated. Being on a twin pac tank. It most likely was used for running nail guns on some construction or roofing jobs.
Borrowed your photo.
The inlets can be switch from the side to the top if needed.
Throttle control tube is missing.
Pilot Valve/Manual unloader. The one on your unloader is missing. They can be adjusted on some models. But NEVER to more than around 40 PSI.
The hex nut on the back side with the hole is the exhaust. Excess air escapes from it when the tank is filled from Maximum (cut out) and low (cut in)pressure while the engine is running.
You can buy new ones like yours for around $50.00.
To be honest. I would not trust any unloader that has missing parts or signs of being tampered with..