I think you're gonna need the manual for that one. Solenoids and rheostats don't go together, solenoids are either in or out. In between and they chatter and quickly burn up. That and the way the lines run on the panel, along with the picture of the variable pump almost leads me to believe that the rheostat may actually be for regulating the hydraulic flow?
All the old trucks were that way, Kenworths we had used miles of white wire with little number tags on the end of the wire. Of course if you had to replace the crimped wire end, you had to cut off the tag to do it. Genius.