OK I better understand now. I don't know where or how you connected but I don't think that bears on your questions.
If it does, we can come back to it. The above diagrams should help you see the arrangements.
My alternator seems to be converted to 1 field by removing the (insulated washer?) and clipping the spade connector. Am I correct when assuming I can reverse this by adding an insulator washer?
It sounds like it may be real unmolested 'single field' alternator. If so, the second brush can not be insulated from housing. The brush holder is cast into the housing. That brush is always grounded and can't be insulated.
