You have the flat reg with the triangle connector with a blue and a green wire to that connector? If so that is the later (70 & up) system & requires an alt with 2 male field terminals in addition to the large "batt" terminal of course. The blue and green wires from the reg would go to the 2 alt field terminals & (switched 12V) from the bulkhead also T's into the blue wire somewhere there but I forget where so that the alt & the reg both get fed). If the original alt died & they replaced it accidentally with an earlier one (with only 1 field terminal) they would have noticed the extra field wire with no place to be plugged in on the alt I would think