As much as I appreciate the ammeter, I don't know why they seem to have been preferred over voltmeters well into the 70s. Even the aftermarket underdash gage sets came usually came with ammeters, not voltmeters. In fact I hav etwo of those in a box in the garage. Maybe accurate voltmeters in that range were more expensive??
Sure would have been easier for Joe average to wire in a voltmeter than a ammeter to his Chevy Nova. (I added a mechanical oil pressure gage to mine)