Need help on 66 engine harness wiring

[ScottH]

I'm in the process of replacing the engine harness that runs from the firewall plug to the starter, temp gauge, coil. PO had bypassed the purple wire to the starter so I've testing the wiring inside the cab.

I've noticed that 1 of the wires on the cab side, the black 1, has constant power going to it. That wire goes to the starter terminal that the positive battery cable connects to. The constant power is present when battery is connected, ignition off, and it is not connected to the engine harness being replaced.

If I'm reading the wiring diagram correctly it appears to originate from the gauge cluster. The truck does have factory gauges, thinking may have something to do with the amp gauge.

So my question - should the black wire have constant power, especially considering it is eventually connected directly to the positive battery cable?

Any help, suggestions, or info would be most appreciated.
TIA