The flange for the cap is on the end of a 2 1/4 inch tube that is welded into the top of the tank. The old style was the flange for the gas cap was built into the top of the tank which was much easier to drain the tank.
This picture shows the weld seam around the tube. I can see the advantage of adding the tube would prevent over filling the tank which would cause the fuel to expand and leak around the the gasket on the cap.
Wm
