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The Corvette parts book lists 3 different left hand AC ducts, 68-74, 74-77 and 78-82.
Other than the round diverter, in the bottom of the 68-74 duct, the 68-77 ducts appear to be the same. An extra vent was added to the lower right front of the driver's side dash panel in 74, eliminating the need for the bottom diverter, so the diverter was eliminated at some point during the 74 model year.
The 78-82 left duct is shaped somewhat differently, likely because of the different dash in the later cars. Also, the 78 up duct doesn't use the metal support bracket, that was riveted to the earlier ducts.
Any duct with the part number 3949987 (68-74) or 336472 (74-77), should be interchangeable, but ducts with part number 14004949 will only fit 78-82's.