Just back from extensive testing on the SAME coil in reference, still in the same car, the '66.
Checked also on the 72 daily driver.
The 72 runs fine with the coil on a bungee cord, the 66 will NOT. This is the coil I "learned" that "lesson" on.
So what's wrong? The meter shows open continuity from either primary circuit connection to the center connector. As soon as the coil is in the bracket, it runs perfectly as it always has. Starts and runs fine. Upon loosening the clamp on the coil, the ignition becomes ragged and motor stops as coil is slid out, naturally providing a few good spark bites to me, but I expected that, even with several shop towels and paper to hold it.
I have been driving with a goofy coil since 1981. No continuity from center connection to case either, so it's been jumping internally to the case all these years.
It worked exactly the same when I put it in new, which was why I had to knock paint off to get the case grounded back then.
I will be changing the coil, and sawing it open to see WTF internally.
Never had reason to suspect I bought a bad coil, so never looked any deeper into internal coil connections, but if anyone is subject to buying brand new things that are bad, it is I.
I ordered a new reissued Hamilton wristwatch maybe 20 years ago, and the face was different from that shown in the advertisement, and I was disappointed, but might have kept it, except that it wouldn't run. Sent that back for refund..
I could list a dozen brand-new "broke" instances if I thought a while, both in personal life and at work.
It is surprising that whatever arcing is happening inside did not eventually burn away the end of the secondary wiring in complete failure.
Let's all learn from this one.