I said initally the red to connect to the stud on C side ( charger ) of the gauge by what I checked on 3rd gen charger wiring diagrams, but as far the pics you have posted your gauge has the RED stud on to the Discharge side of the gauge. So I was wrong saying you what I said on this.

The ammeter has a counterweight inside moved magnetically with the load going through what moves the needle. So maybe the diff with my assembly is due the diff gauge design and counterweight location/indexing.