Doesn't look like a relief valve .
Looks to be a port, maybe for a gage.
Is there a lever on either valve? I don't see one.
I see OUT port and PBY.
Normally, you don't see a PB/BYD port plugged, as it is supposed to feed the next valve.
Is there supposed to be a jumper from the BYD QD to the OUT QD.
The other port probably goes back to the 3pt.
The OUT port probably goes to the side of the tractor sump.
This looks like a set up for a BH or a log splitter.