The voltmeter fitted from '76 up to '83 was a thermal one, very slow moving and crude in its reading. Not coincidentally it was fitted on other french cars of the time-1st generation 5 Alpine was criticized for having this pretty useless voltmeter but no temperature gauge! Sounds to me that is something having to do with either their instrument suppliers (mostly Jaeger) or alternator / control box suppliers.
In my opinion, any car should be fitted with a charge warning light (insead of voltmeter) and a low oil pressure light (insead of gauge). You can't have your eyes pinned on the instrument panel all the time, and all you want to know is that the battery has stopped being charged or that there is no oil pressure.
You can substitute the water temperature warning light sensor with a gauge sensor, thread is M18x1.50. Most aftermarket water temperature gauge kits come with an external ("contact") sensor, which is not as accurate but is often the only possible solution. If you already have a temperature gauge make sure the sensor you will buy matches it, different makes have different operating resistances.