For what it's worth, if any, I read:

"Adjustable" third brush generator uses a cut-out.
"Fixed" third brush generator uses a matched voltage regulator.


Here it is from vccachat:

The 1938-39 passenger car generator is different from the truck generator. ****The truck used the old 1937 car generator that did not use a voltage control.**** Later there was a Delco Universal generator sold that covered all applications from 1929-39. Its use required different third brush settings and grounding of fields and should not be confused with any of the many generators used from 1929-39. The proper 1938-39 car generator is a 1100004.The universal was a 1100460, later changed to 1100502 in new and 1100503 in rebuilt. These universals are basically a 1939 unit that can be adapted to all applications but not the same as the original 1100004 unit. Among other things it's third brush must be adjusted differently for the various applications and a pully with a fan must be used for the higher settings.

So, the options are:

* To be original, then what's in the truck now is wrong. Need correct generator, cut-out, light switch, etc.
* To keep the 1100002, then a Delco Remy 5814 (or equivalent?) is needed. If the generator brushes are worn etc., needs a rebuild.
* Update to more modern 6 volt generator and regulator.
* Update to more modern 12 volt generator and regulator and do a complete 6 volt to 12 volt conversion.
* Update to modern alternator and do a complete 6 volt to 12 volt conversion.

I'm done.

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."