The side vents in the dash ("Astro-Ventilation") had nothing to do with whether the car had a heater or not - they were on every non-A/C car, and admitted outside air from the plenum below the cowl vent grille, controlled by the upper of the two vent pull knobs on the kickpads. On A/C cars, they were part of the cold air distribution system; all cars had the outlets in the ends of the dash.
I'd think that the dummy cover for the heater control opening would be almost impossible to find, as only cars shipped overseas or to Hawaii could legally be built with the C48 less-heater option; federal MVSS regulations applicable to domestically-delivered cars required a defroster, which required a heater.
