If I remember correctly, on the 41-47 series engines, the distributor was not naturally grounded. The reversing switch controlled which way the current flowed through the points, so that (theoretically) there was an "even" amount of wear on each side of the points when they closed. (as is, because the current flow is all one direction, one side of the points wears out faster than the other side - I'm sure there's some fancy name for this phenomenon - "electro-carbon-fracturaization" or something! ;-) With the reversing switch, each time the truck was started, the switch was flipped mechanically, so that the direction of electron flow flipped directions, thereby causing even point wear, and longer-lasting points. Theoretically.
Eventually, someone at GM came to their senses, and realized that this over-thought solution was more expensive than the problem it "cured" in the first place. GM published a bulletin explaining how to take the switch out of line, and add a grounding jumper to the distributor.
Or so I assert, anyway . . .
-Michael
