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.
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.
