Hi Phil, and welcome. could you give us a bit more to work with please. What was, or is your Brush generator driven by? Does the unit have a starter motor or is it a Start-o-Matic type?
The SOM variants have a starter motor/dynamo built into the generator head. If you look inside you should see a couple of brass slip rings and also a commutator (segmented) ring. These units have a main field, a series field and an auxiliary field.
Does it look like this photo inside?
Bob