Also, the 2500MM originally had a type 35 dial that WAS polarity sensitive, but could be upgraded to a 2500MMG with a type 71 dial that did not care about polarity - can you find a code number on the dial with either 35 or 71 in it?

Correct me if I'm wrong, but I think that the Red (ring) wire should be negative 48 volt relative to ground and the Green (tip) wire should be close to Ground