squareback alternator wiring in pre 70 car issue

It is not a second field. Both early and late style alternators only have one field. The difference is that on early alternators the second end of the field is grounded internally in the alternator . Later styles with the solid state voltage regulators ground it in the regulator, thus the second field terminal. It doesnt matter which end of it you ground to use it in an older car.

Have you ever had your volt meter calibrated? Have you tried a different battery in the car? As was stated, drive it for a bit, it could just be trying to top up the battery or trying to charge a battery with a bad cell.