OKAY I think I've completely figured it out now (at least in my head anyway).
Building on what I just mentioned, I
think i've figured out why my previous attempt failed. What happens is that when you tighten the screw in the photo, it causes the barrel to to move around. Even a few mm of movement means that it is no longer aligned and the car won't run.
I noticed though that there are two notches that you can use for reference. I confirmed the position by again using the safety pin to push through both holes, then tightening whilst keeping the pin in the hole so that the safety pin keeps the barrel in place.
The distributor has the Shellac gasket, all springs in place and now the metering hole aligned.
If this fails I GIVE UP LOL. I have a really good feeling now though
The empty box is where the barrel notch should lie. Just slightly left of the slit in the distributor. Use the safety pin to ensure it is completely aligned though, but at least now your adjustments are within a a couple degrees of movement instead of 355 degrees of movement
I replaced ALL O-rings using a Nitrile O-ring kit from Harbor freight. It had all sizes needed!
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I got it from the junkyard, but took a good cleaning.