Starting circuit challenge - intermittent fault SOLVED

Staying in that vein.

Based onthe Aus. Valiant Wiring diagram, this was probably the fusible link
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A couple ways a fusible link can go.
The way they go quick is if an unprotected circuit gets shorted to ground.
This could be Joe fat finger reaching behind the instrument panel or removing the alternator with the battery disconnected and a wedding ring or spanner contacts the hot and ground (earth). Or in a blue moon something in the alternator output insulation fails and poof there goes the link.

BUT. it would not to follow that line all the way to the bulkhead and then inside the car.
Same on the alternator output line.
See that they and the main feeds to the key switch, the fuse box, and the headlight aren't chafing against sheet metal.

A somewhat common way to slowly deteriorate a fusible link is to run the battery down and then charge it from the alternator while driving down the road. That causes high charge rates for extended ( 10 minutes) periods of time. There will be heat damage at every connection between the alternator output and the battery.

Looking at the condition of the other wires I'm not saying either scenario I just mentioned is what happened. But if there is short lurking, best to know it before it bites ya.