[Ramtuff]

The reason for 2 nuts is simply one to hold the stud and isolator in place and one to clamp the cable.( are you certain that’s the ground?😀)

 

[Ramtuff]

The plug is the field control for the output and the large cable attached to the stud is the positive to the battery.
Th ground is made by the fasteners bolting it to the engine.