In those pictures, the forward line is the fill line. The rear is the overflow. That airplane happens to have a uniflow setup in the tank. So it was a 3 line tank to start. So the overflow line is simply a straight line back to the tank pipe that is the overflow pipe inside the tank. The fill line you see goes to a T in either of the other two lines. I'm not even sure if it's the pickup line or the pressure line.

To fill, I pull both lines out like in picture 3, pull the two plugs and fill through the front line. When the back one spits, I plug the two and push them back like in picture 4.

The plugs cost something like 50cents each. I'd make my own, but why?