You got that right. Very frustrating as you start doubting everything else associated with it, like the wiring, a hidden intermittent short, a poor ground, etc.....
One never suspects a faulty new/rebuilt part until he's tried everything else in his trouble shooting arsenal. Unfortunately, it's wasted time.

Looks good. And there's nothing more frustrating than a new part which is defective

I replaced the voltage regulator. Started the car, and it lasted about 3 minutes before the voltage crept up and stayed at 16.5v. Got it checked and the shop said there was an intermittent short on the stator coil. Got a Bosch replacement and it runs with no more spikes and drops. What a nightmare when you have a rebuilt dud.
For the jumpers, I used the plug of an old external regulator with waterproof spade connectors for troubleshooting if required.

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Based on the troubleshooting provided in the FSM, it's indeed the IC voltage regulator.
Now to source one locally. The alternator repair shop should stock these, I hope.
Only 5 pins to solder.

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Thanks for the parts link.
Much appreciated.
Happy New Year.

Sure you can buy the regulator -- see our tech article Hitachi LR Alternator

Thanks for the troubleshooting steps.
I will check it later today and confirm if its the IR.
If it is the voltage regulator, I doubt that they sell them as a separate part for the Hitachi, meaning another re-manufactured alternator might be the cheaper option to go with rather than bringing it to a shop, having it checked and then repaired and getting hosed for labor.

Next I would check the output, not at the battery, but at the BAT terminal on back of alternator. Start engine and check red wire to BAT terminal, black wire to alternator case

Then check (while engine running) the voltage at the T-connector (with connector plugged in). When under load, it may differ from the disconnected voltage, and if so that indicates a connection problem (bad solder perhaps)

Bench test the regulator is rarely done -- I've never seen an internal one go bad. But basically you'd unsolder it from the alternator, connect + voltages (reference and sense) and ground, then check the Field terminal for voltage. Then raise the supplied voltage to over 15 and verify the Field terminal shuts off

The Datsun 210 1979-1981 factory service manuals have a troubleshooting method, in the section Electrical System. They specify check wiring then shorting the Field to ground by sticking a paper clip in the hole on the back of the alternator. This is done with alternator on car and wiring connected

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I just rechecked the wiring and they're all OK.
The dash light checks OK.
The rectifier checks OK.
12V at both T-connector wires.
It must be the internal regulator. Not sure how to check that on the bench.

If it was my car, i'd re-check the wiring. With engine OFF, key ON, pull the T-connnector out of the alternator and check for 12V at both wires of the T
* 0 V - wiring problem. Perhaps the Fuse Box. Or maybe the dash light burnt out
* 12 V - good sign