[special-K]

Yeah, that's not a factory set-up. Sorry to disappoint, just a fact. I was willing to learn something new but nothing new here. Those tanks could have been ordered out of the Silver Book (GM approved upfit catalog) and installed before original owner took delivery. Even financed in the car loan. The truck would have been received by the dealer, then set to a local truck upfitter or possibly installed in the dealer service dept. The only factory option side-mount auxiliary fuel tank offered prior to '73 was in '72. It filled up near the top of the bed near the front with the same type cap as the tank in cab and there was a sheet metal cover over the filler tube between the top rail and floor. I have seen aftermarket plastic plaques for the tank valve and it may be that it was the same company as the factory jobber.