There were two Harrison heaters available from the factory, and an untold number of aftermarket units, since the heaters were a dealer installed accessory. There would be a recirculating heater with a round core, and a fresh air heater with a square core. The fresh air heater pulls air in through the louvers on the passenger side, and the motor sticks through the round knockout in the firewall. The recirculating heater mounts differently and doesn't use either knockout.
