1984 Ram Crank No Start

I'm not understanding a few things:

I have a 5 pin ECU, the plug has 5 female pins but only 4 wires, why?
I have a single ballast resistor, factory appearing harness, is this correct?

The biggest hurdle for me is I see no cause and effect, or maybe the reverse, for everything I have checked.

For example, I disconnected the coil wire and tapped to ground, coil fires a mad spark, but I have no idea what that tells me.
I've seen it suggested multiple times in various threads, but it is not clear what it means, the coil is good, the ECU is good, etc.

I don't know what each thing I'm testing is eliminating as the issue, or what it should lead me to test next.