Its a rectifier bridge for the 140 amp alternator. Thats usually the part that goes. Notice the three studs. They each have two brass clips under them with diodes attached. When the diode goes the current goes in both directions and the alternator shorts out (dead). The long life brushes are good and the bearings just nead to be cleaned and lubed.
Problem is finding one of these things at a reasonable price. $20 form an automotive shop supplier but they only want volume orders.
One parts store quoted me $180!