"Later I received a package from Terry's Jaguar.
Installed rotor button.
Fired her up. Idled until warm. Test drive. All good.
How does a piece of metal mounted in plastic fail in such an interesting way? Looks ok to me..."

Not sure what a rotor button is. But, assuming you installed the new dizzy cap you ordered as well as the rotor arm, it is far more likely the fault was in the dizzy cap and the little carbon brush in the top of it that connects the king lead to the rotor arm. If you did not install the new cap yet, then I feel you should do ASAP, as you probably have an intermittently faulty old cap, and it may well let you down again. Anyway WELL DONE, you have fixed it yourself, worked through it sensibly and also made other bits of the system more reliable and well serviced. Great stuff.
Greg

 

Southern shade tree mechanic talk. Its a Rotor, or a Rotor arm ( JLM9624 ). And, as strange as it may seem, it is faulty.

I did not replace the dizzy cap yet. I will, because I have a new one and there's no reason not to. But I am trying to be methodical. One thing at a time, test, then move on.

The car would not start at all anymore during my Sunday work. Despite having good spark to the dizzy cap, checked with my spark tester. I suspected the cap too, as it could have any number of issues that could cause a failure. Carbon tracings, cracks etc. While I can't think of any possible reason for the rotor to cause this.

So new rotor in and it runs! But I had to confirm. Old rotor back in. Will not start.

I know. But that's science for you. It doesn't care what I expect.

EDIT: I may have found the fault in the rotor! Inside the rotor where it slides onto the dizzy shaft there is a coating of rust at the bottom. There is a very small hole at the very bottom of this shaft-hole and that hole must go through the plastic to the bottom of the metal bar. What I assume is that the rust in this hole was carrying the current through the plastic to the dizzy shaft. This rust wouldn't be a good conductor, but apparently easier to jump through than the gap between the rotor and dizzy cap pickups. I'll meter it out tonight to see if it's connected to the bar, then de-rust it in phosphoric acid, clean it well and re-test to see for sure.

EDIT 2: No conductivity between the rotor bar and the rust inside. Not surprised there. But what I thought was a small hole in the plastic appears, under closer inspection, to be a scorch mark.



Scorch mark in rotor.

 

That rust track I have seen ONCE, on her PreHE, and like you, it took me weeks to find it. That was pre computer days, so method was the madness.

Good find, and glad the beast is running.