Ammeter voltage

Amps = current (flow rate).  Voltage (pressure) is not relevant. So there's no need to change it. So you can pick whatever you like.

At 6v, the same power needs more current, so logically an ammeter with a wider range should be needed. But charging systems weren't as good in those days. 

Some modern repro units come up a different size and will not fit.

The voltage just signifies the range on the face based on the charging outputs, 6V 8-0-8 12V 12-0-12, both can be fitted to either voltage and will measure amps equally. After the first few miles the needle will be swinging so violently it won't matter, especially after the needle has dropped off,

"The amperage indicated is not a prediction, its only a scenario"

I like it Mike. JH The numbers on the dial of the ammeter do NOT indicate Volts they are just Amps. As you say Mike the Amps are some-were here abouts! If you really want to keep a check on your charging system then an LED volt meter is a better bet. Any one who tries to sell ammeters on Voltage just shows they do not know what they are selling.