What Does This Black Fused Junction Block Wire Do?

[Gregski]

Apologies for yet another: "what does this wire do question" LOL. I did pull down all the diagrams I could find and my truck is running and driving fine, however I am perplexed so I ask the question.

So what does the wire Arrow #1 in the diagram below do. Looks like it may be labeled "Engine Harness Connection" I ask because it does not connect to anything on the inside side of the firewall bulk head connector, Arrow #2.

I put this '71 GMC basket case truck together one piece at a time, so that's part of the reason I don't know this.

Full disclosure I converted to a one wire Alternator setup and again truck runs and drives fine without this black wire as I have deleted it, but I am just asking what was it for?