I was wrong, my '38 RC ammeter also has 3 posts. I used the fused post to run power to a fuse block from which I run headlights/taillights, dash lights, wipers, heater, and radio from. Also changed over to 12 volt.
My shop manual indicates that one unfused post is connected to the power stud on the starter, the other unfused pole is connected to the generator and relay, and the fused post powers the headlight/taillight switch, the horn, and the instrument panel light switch. The manual doesn't indicate what gauge wire was used on any of those circuits.