They were DEALER INSTALLED for customers who pulled small camper trailer, boats, or whatever. People in the rural Midwest used their cars for more than just a way to get from point A to point B.
I don't show any in the parts manuals I have. Maybe it was like the aftermarket trailer hitches or aftermarket wheels. I know our dealers would install aftermarket goodies for the customer and add it to the sale price so it could be financed along with the car.