From 64 till 65, the rectangular donzi style clam shell vents appear to have been optional. However all donzis of this period have a functional Louvered Vent in the Center of the Deck, just above the transom. They all also have a Register Vent in the base of the rear seat.
From 66 to 68, the stern louvered vent, is non-functional, and venting is thru the Register Vent and two of the standard donzi clam shell vents.
From 69 to 74, when the HIN's became imprinted, the stern louvered plates and the register vents were eliminated and all have four of the clam shell vents.
Post 73, the last digit in early HIN numbers will be the year of manufacture. Later HIN numbers use two digits for the year.