Intermittent Ignition Issue [66 Valiant /6]

Still no idea what you have in front of you.
We don't need to see a diagram. You need to show us what you are working with.

Without that, best we can do here is explain how it is supposed to work.

First. Before any testing, slow charge the battery with a charger.
12.3 Volts with nothing on is on the low side.
With the starter engaged it should be above 9.5 Volts. This is a load test and reveals the charge state and condition better than measuring with no load.

Starting sequence illustrated here.

How the starter relay works.