Does your generator have three brushes? A third brush generator was used by Delco Remy through 1938 or 1939 on Chevrolet cars but my books don't specify for trucks. The third brush was used to control current to the field windings and regulate current output at higher engine speeds. It took advantage of the inherent distortion of the magnetic field due to rotation. The maximum current level may be increased by moving the third brush in the direction the armature normally rotates. The faster the rotation, the more distortion so the brush was moved to a position to pick off higher current for the field windings at the RPM the generator usually turned. On a truck where the engine is running at higher RPM more of the time the third brush position would be critical to avoid overcharging the battery and shorten lightbulb life. Of course if it's set too far the other way the maximum current output might not keep up with the lights, heater, etc. Also setting it too high could overheat the generator to the point where the solder melts out of the commutator and the windings come undone. I had a Lucas generator on which this happened. Hope this helps.
Generator number Output @RPM Field Current @ 6V
1937-38=948-R * 20A 3400 2.3-2.6A
1938-39 = 1100004 28A 3600 2.3-2.6A
1940-48=1102667 35A 2400 1.75-1.9A
*Third brush model
All rotate clockwise
All except third brush model voltage set to =7.4V
Current and voltage regulator air gap =0.075"
Cutout relay air gap= 0.020
