Voltmeters did not start being used on production vehicles until the mid-1970's or later. All the stovebolt trucks had either ammeters or idiot lights to indicate the performance of the charging system. Some people who don't understand charging system operation very well prefer to install a voltmeter, thinking that it will give a better indication of the health of the charging system.
In actuality, it would be necessary to have a voltmeter, an ammeter, and a method of monitoring the specific gravity of the battery acid to make a well-informed evaluation of the charging system, plus enough knowledge to interpret all those readings. That would take virtually all vehicle owners, and at least 90% of professional mechanics out of the equation!
Jerry
