If you are going to get a Voltage Regulator get the correct one for the Alternator you have.
The below works on the Stock Alternators.
If not, set your meter to Volts AC and you should have a reading of ~100mVAC.
If the VAC is too high, then look for a rectifying diode failure. If VAC is OK but the VDC is not correct, then the problem is with the voltage regulator.

If I accept the fact that on My W123 I need to have a Bulb (2 watts) in My Charging Warning Light circuit in order to charge correctly what would happen if I installed a Volt Meter in place of the Light?

I would check and see if the SAAB 9000 has a Charging Warning Light or a Volt Meter in it and try to duplicate that circuit.

I had an issue with a non-Mercedes Bosch Alternator and took it to PepBoys and when they checked it they gave me a little receipt sized print out; in My case on what failed on it.

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84 300D, 82 Volvo 244Gl Diesel