Curiosity got the better of me so I fiddled with it this PM. The hex part is actually a cap, underneath is a jamb nut and adjustment screw.
I also happen to read the TM on crane maintenance earlier and it referred to checking hydraulic pressure at the valves in the gondola by installing a pressure gauge and then removing the acorn nut on the RHS of the lever control body. You then loosened jamb/screw combo to adjust pressure up or down.
So using the logic that control valves are somewhat similar in nature, I think the comments on relief pressure are most likely correct and this is the area where the drag winch is 'turned down'.
only thing is both the directional lever AND the torque lever have one, so could be one or both are adjusted down.
i might enquire at some local hydraulic shops to see if they have the equipment to test the pressure.
Does anyone know what the standard pressure was on 936 -a1's at the drag winch? I know for the crane it calls for 1350psi +- 25psi...