There were "once upon a time" longer and shorter rotor contacts. I'm not sure that much would make a big difference if the rest of the system is right. Just FYI, "15 ish" volts is not correct. Didn't I post above how to check this?
Another thing you might check is for alternator AC ripple. With the car running and battery normalized, put your meter on AC volts and clip it to the alternator output stud. Recheck with lights or other loads turned on. You should read VERY little, maybe a few tenths of 1 volt
Did you ever try to run it with the alternator field disconnected?
