The joys of wiring....1965 c10 harness blues

[lha1992]

Hi all, well been working on my 65 c10. Yanked the original 283 an powerglide outta it with the help of member dillon rock on here. The truck is a original 283 powerglide custom cab BBW with all the gauge instruments in the cluster.

So I was doing some wiring an was planing on rebuilding my engine harness with new wires going to the starter an coil an because I'm getting rid of points. Well heres the thing the truck never would start on its own from the key so you'd have to jump it from the positive side on the battery to the s-terminal on the starter with speaker wire. I figure maybe the harness or the neutral safety switch which I have another mystery about that coming up too.

Anyways theirs 4 wires coming outta the big plug on the end. Purple is to the s-terminal, green to my thermostat gauge sender, that weird colored cloth wrapped one is the resistor wire I imagine which goes to the r-terminal on the starter which then in the background you see the yellow wire that comes to the coil from the starter BUT what is the black one???????

I'm dumbfounded as to what it is an kept looking at the wiring diagrams for awhile. Either I'm clueless an not realizing it but WHAT IS THE BLACK WIRE?