Weird NA Mk1 starter motor fail issue

Well, never seen this before. A well maintained, low miles, and excellent condition Mk1 (Eunos Roadster). Turn the key for the first time in ages, starter engages briefly then nothing.
It’s like the problem you get when the battery terminal isn’t secure…
But it wasn’t that, much chasing, swap battery, then thinking immobiliser - but you don’t hear the solenoid engage with that problem and LED said it was off.
Anyway, problem found and it needed a new starter motor… Posting here to help anyone else as this was a sudden unexpected failure of a part I would never have checked. The braided connection between the solenoid and starter body has vanished! I guess some weird sort of corrosion, I don’t believe it’s ever been apart before.