[kiwidave]

Solenoid wires are one to battery (at the large-diameter long stud), one to starter motor (large diameter, short stud) and one to starter relay (the small post). There shouldn't be another one there...maybe it is an extra earth from the small post.

I say that because although I've never thought about the solenoid's ground....careful looking just now at a few Shovel era factory manual diagrams indicates that it is grounded by some apparently internal connection between the small (starter relay) terminal and the casing. So... self-grounding if bolted up correctly. Like a regulator. Apparently.