Fuel Tank Control

[TXBowTie]

I got my '75 C10 2WD just earlier this week. It has dual tanks, but I don't understand the tank switch. I have a Chrome T-handle knob on the floor near the left front driver's seat, that I can push down or pull up, and a black, round knob on the dash (near the glove box), that I can twist to point left, right, or anywhere in between (I am not sure it is even connected to anything).

How/what are the factory controls I should be looking for that would control which tank I am running? Is there an easy way to determine which tank I am running currently?

The fuel gauge doesn't work (of course), but by tapping on the tanks I can tell that both are nearly out of gas, and I don't know which one I am running on (would prefer not to fill them both).

Thanks guys,

Wayne