The 948-R generator does have a third brush, however, the 1937 shop manual states..."NOTE: DO NOT ADJUST THE THIRD BRUSH ON THIS TYPE GENERATOR" (This is about the only time I have seen capital letters in this manual). It goes on to say that the brush is set at the factory at the maximum safe output and this setting should never be disturbed. There are 3 resistance sizes available for the field resistor that goes on the back of the light switch. Normal is 1 ohm and you can use 3/4 ohm if the battery does not stay charged or the 1 1/2 ohm if the battery becomes overcharged.
Kent
