AWOL voltage regulator.

That VR could be wired anywhere on the blue wire that goes to the ballast resistor. Anywhere. That blue wire proceeds to the ignition switch. So anywhere includes under the dash.
That does look like an isolated field alt. And it looks to be full fielded.
The proof is in the voltage. Measure the battery rest voltage with the key off. Then start er up, and read the running voltage. If it is higher than it was, then its charging. If its within 1 to 1.5 volts higher and less than 14 thats good. Rev it up a bit. If the votage stays about the same, theres a VR somewhere. But if the voltage climbs to 16 or higher, the the alt is in full fielded mode, and needs to be rescued ASAP, to prevent system meltdown.This doesnt mean that the VR isnt still hiding somewhere, only that its not in circuit.
Are you familiar with what the VR looks like?