You should be able to use either the Henry ladder sight (the one Major 2 referneced) or the '73 carbine ladder sight. Some of the earliest '66 rifles used the Henry style, then they changed to the '73 carbine style (shorter ladder), then to the semibuckhorn. A lot of people are not crazy about those ladder sights since they are not elevation adjustable for shorter distances (flipping up the ladder is mainly for longer ranges) and so they change them out for the semi-buckhorn or similar. So if you ask around you might find a used one cheap.