[RussWhite]

I guess we are all entitled to our opinions here, so this is mine....

Field current is supplied to an alternator through brushes riding on slip rings. The more current that is supplied, the more voltage the alternator produces at the same rpm. It is the job of the voltage regulator to determine how much current to send through the field to produce the appropriate voltage.
The field circuit is a series circuit. In other words, the current that goes into the field coil is the same current that comes out the field coil. That current goes in through one brush and returns through the other brush. Much like a battery, the current that leave one pole returns through the other. You open a battery on either positive or negative side, all current stops. Same with the field current. You have one bad brush not making contact, all current to the field is stopped and the alternator will not produce.
So my opinion is, when the one brush became worn to the point of no contact, your alternator stopped producing the correct voltage. This happened because one brush was not replaced when the alternator was rebuilt. The story was fabricated by the builder to cover his mistake. Brushes wear at the same rate as they have the same current going through each all the time. That's my theory. In any event, you should be fine now with good brushes on both poles.