the fiberglass part of the box is the same from 1966-1970. 1966-67 has some differences with the heat/ defrost flapper and a seperate part number (MODEL 747). 1968-1970 are all the same with 2 slight differences. first the part number. and, second the reason it is a different part number is the heater core. 1966-1969 is the same heater core with a 5/8 inlet and a 5/8 outlet. the 1970 has a 1/2 inlet with a 5/8 outlet.