This early set-up is different than the later system.
In the later models (up to 1986) only one common sensing leg (pink) was run from the gauge to the control head on the tank select valve. Two leads (pink/blk & pink/wht) are connected from the control head on the valve and run over to the individual tank senders. One of those branch leads is always in contact with the common pink feed.
When the tanks are switched, the valve shuttles. Attached to the valve - the part that shuttles - are a set of contacts. They move as well - and that changes the connection from the pink to either of the branch leads.
So, when the dash switch is moved, the valve shuttles. And along with the valve, the contacts set also shifts. That changes which tank level sender is connected to the sensing leg of the gauge.
In the earlier version (like yours) the select valve is much simpler. It only changes the alignment of the fuel pump suction piping from one tank to the other. The sensing leg for the gauge (tan) is split off into two leads at the dash switch. Each of those two leads (tan & light blue) are then run independently all the way back to the individual tanks.
So, as you change the position of your dash switch, two things happen simultaneously - independent of each other:
1. Power from the fuse block (via the pink/black) to the tank select valve (on the light green) is being switched on & off.
Your type is a solenoid actuated valve. Without power, it moves to the default position - and connects the fuel pump to the production tank. Then, if the switch is moved to the closed position, power is applied to the coil. When the coil is energized, that causes the valve to shuttle - and the fuel pump piping is connected to the auxiliary tank.
2. Incorporated within the dash switch, are a second set of contacts that are shifted (along with the on/off contacts for power to the solenoid coil). These contacts determine which of the tank senders is connected to the dash board gauge.
When the switch is in the production tank position, there is no power to the coil. Also, the tan wire from the gauge is connected to the tan wire that runs from the dash switch to the production (LH) tank.
If you change the position of the switch to the auxiliary tank, the coil is energized. At the same time as power is applied to the coil, the gauge sensing leg is being shifted. The tan wire from the gauge is disconnected from the production tank sender (the tan) and connected to the auxiliary tank sender (via the light blue).
Power to the gas gauge comes in on the pink 39 lead - just a common (ignition switched) hot lead.
