Help with 5hp 2 stage compressor external valves

[Rcs]

Trying to put a old Beaird compressor back in service ,new 5hp mtr and the compressor I think is a champion nr 14 6 , 2 stage but one of the e terminal valve lines is missing , i am not sure about the valve function or where to hook up a new line.Have pics

 

[Bill C]

That is an auto drain valve. It is missing the plumbing to the bottom of the tank. It is triggered by the air coming out of the unloader valve on the compressor. So basically every time the compressor cycles off and the unloader discharges the pressure in the cylinder head, the tank drain opens briefly to drain any moisture.
Take a look at the Champion ZTD-1 auto drain and study some pictures on the web to see how it should be plumbed.