Alternator polarity?

It would appear that you have a square back 2 field wire alternator, Correct?

I would disconnect the 2 field wires and check both field wires to the case of the alternator with an Ohm meter, you should have no connectivity from the terminal to the case. test each terminal to case seperatly.

My gut says that one of the terminals is grounded (shouldn't be for a square back 2 field wire alt)