C50 wiring in a C10?

[rcg66]

I discovered a bad wire in the underdash harness on my 66 C10 Longbed Fleetside, and when I was looking up the wiring diagrams, I ran into a puzzling situation. The engine harness connector only had 4 wires coming from it. One of these wires was black (the wire that was bad).

All the wiring diagrams I could find show that there should be 5 wires with none of them being black. There was also a black wire with a white stripe coming out of the front extension harness connector which none of the diagrams show also. After looking through my assembly manuals I found that the wiring harness I have in the truck would actually be for a C50. To add to the mystery, I have the underdash harness out of a 66 stepside that I parted out and it also only has the 4 wire configuration with a black wire and the black with white stripe wire that matches the C50 diagram.
I don't have the harness that connects to it that runs out into the engine compartment, so I don't know where the wires ran after that.

Has anyone else ran into this situation before? With the way the harness was mounted and ran, it doesn't look like it was ever replaced.