The sending unit is grounding through the lock ring, fuel tank, tank straps. The factory added the strap so the gauge would still work when the lock ring, fuel tank, tank straps rusted and stopped completing the ground. I bought the short strap while restoring my 73. I needed the long strap. I haven't gotten around to installing it yet. My gauge works too.
The post that the sending unit wire attaches to is isolated from the sending unit body with a gasket.