Mine is marked as RH (up) and LH (down). In early trucks the right hand is the main tank, so with no power to the solenoid under the truck it will run off the right hand tank. With power applied the solenoid switches to the left hand (drivers side) tank.
The DPDT switch serves two functions - activate the solenoid, and switch the gauge feed over between tanks. One “side” needs a power feed from the fuse panel, and of course the power line running to the solenoid. The other side has the gauge feed in the middle, and the sending unit wires on each side.
I assume your truck has the single power terminal solenoid like my truck (later trucks have a six wire connector). It may be a 3 port solenoid that only switches the fuel feed, or it may be a six port that switches fuel feed and fuel return. The old six port, single terminal solenoids are impossible to find (all the new single terminal Pollack valves are 3 port), but I’ve seen postings about how to switch an old truck to the newer six terminal solenoid.
