Lets start things with welcome to the board!
That picture you found is incorrectly labeled, so here is a corrected version from an older post of mine.
The terminal in question is a battery power feed.
If you don't have them the factory service (wire schematics) and overhaul manuals will prove helpful and can be downloaded at this link.
http://67-72chevytrucks.com/vboard/s...d.php?t=558016