that is one heck of a compressor. your lucky to have 3 phase in your shop.
with all the shops around here going under, used 3 phase stuff is cheap now.
that tube may be an clutch type unloader.
some of those bigger units were set up to run the motor continuous
and clutch the compressor in and out.
this avoids a lot of restarts of the motor under high air use.
our shop has a 7 hp 2 stage that puts out 34 cfm @ 90 psi.
we can set it to run all the time by opening the intake valves using a regulator.
never have, we don't use that much air.
it looks like your compressor has 4 pistons; twin 2 stage compressor heads.
might be 50 cfm @ 90 psi. that's a whopper.