Hi Dave, the red terminal in the top pic is battery pos not earth.
I’m assuming your 12.5V and 13.3V readings were made with the meter connected across the battery terminals and so are correct.
I’m reading it that you then took off the meter probe from battery neg and put it on that red alternator terminal thinking it was earth and expecting the same 12.5V and 13.3V but you got 0.12V.
You then put it on engine earth after the light had come on and got the 17V reading.
If that’s right, the reason for that 0.12V reading is that both probes were connected to pos; one near the battery and the other near the alternator. The 0.12V meter reading is the voltage drop across the resistance in that pos loop and is about what we’d expect. When you transferred it to engine earth you got a proper reading and saw the engine light was on and it was at 17V.
To double-check that red terminal connection, leave one meter probe on battery neg and put the other to that red terminal and you should get battery voltage.
So our diagnosis is unchanged. Check again but it seems that the regulator works at first then breaks down and allows 17V - 19V instead of 13V.