as Tim points out, there's a difference in Regular Production Options, normally installed during "production", and Dealer Installed Options, which may have been either retrofitted RPO's or Accessories
and there's 2 categories to consider here, vehicles that were sent to the dealers "on spec" and vehicles that were ordered by the dealers, either cuz they knew what would sell in their market or cuz an individual customer ordered it - those 2 categories would have been different in different areas of the country, remember there were a dozen "factories" producing trucks for different areas .... not many trucks produced in Los Angeles or Oakland for dealers in SoCal or AZ would have had factory installed heaters, many produced in Flint likely would have had the standard heaters, with some percentage sent to dealers that sold more "upscaled" vehicles having the deluxe heater, and of course some "base models" with no heater
Bill
