I am going to throw another monkey wrench into this discussion.
The original tanks were not painted, nor were the tank straps.
Given this, the tank would have been grounded in effect.
So why did they add a ground wire under a sending unit screws on one end while the other end of the ground wire was placed under the cab, under a preexisting nut and bolt which held a wire clamp.
Maybe they had issues with the tanks remaining grounded early on and this ground wire was a bandaid fix implemented an all trucks from that point forward.
It would not be the first odd bandaid fix implemented on AD trucks through the long run of production.
