Funky alternator wiring- '73 Duster

The wiring in my Duster has been confusing the heck out of me, well the alternator wiring to be exact. All of the correct alternator wiring is there however there is one extra black wire. It's a thick one. I originally thought it was an extra wire that ran along the alternator harness but it's not, it comes out of the harness. It dosen't go anywhere towards the alternator either. It just comes out of the harness and then it's capped off. I've been looking at other cars at shows but I don't see the wire anywhere! Any idea what this wire could be?

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