Found my dads original 1971 duster!

No. The ring terminal was fine. It was as if a wire touched there or something, at some point
H',m interesting.

A loose connection is my only guess right now since when you directly connected the battery wires the car started and ran.
Since we're not confident of the where the break was, it wouldn't hurt to gently tug and push on the wires at the bulkhead connected to see the terminals aren't loose. The terminals have barbs on them sometimes they don't catch or hold.

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