Dumb Generator Question

[Bruce Parker (RIP)]

My generator is not putting any current out (it had supposedly been rebuilt some time ago and has not been run since).

It is a 12 volt General Motors 4 brush type. Taking it apart I find two brushes (marked 'A' on the photo) are attached to the armature and only one (marked 'F') to the field. The last brush has no wire attached to it and does not appear to be 'grounded' to the generator body. So, my dumb question is what should attach to the 4th brush? Nothing? (assuming the last guy who had it apart knew what he was doing), a wire to the 'Field'? (to match the one opposite it) or a wire to ground? The manual says to make sure you carefully re-attach the wires as they were, which is a little help at this point.

I'm hoping it's as simple as this and I don't have to rig up a 110 volt growler or other dangerous looking apparatus.