Got the alternator back today and it turns out that the internal leads to the diodes and the stator were broken due to the alternator ingesting some foreign object in the past. Who knows whether it was a giant bug or a loose screw sometime in the past 38 years, but now it works again. The alternator repairman replaced the rear bearing, the brushes and soldered the connections back, and now it puts out 55 amps. It cost $80.25 total.
Oh, happy day! Now the amp gauge actually goes to the right and not just the left.
