Hi,
Your pictures look very familiar. This is because I have the same year and design of heater in my 63 c10. Most 60-66 chevy/gmc trucks had 2 designs of heaters.
Deluxe: where the heater core and motor were in the engine compartment and the air box when in through the firewall into the cab. Most the these heaters had cores got pretty leaky, since they were on the outside of the truck. They also had more levers on the dash panel to control options of heat, hence they were "deluxe" in standards of the 60's.
Recirculating/Fresh Air: Is where the hole system is inside of the cab, under the dash behind the glove box. These systems had the heater core inside the cab, and usually a greater percentage of these that remained original had a good core last longer since they were inside the truck. The heater air box, core and motor are all in the cab underneath as well. They also only had 2 levers on the control on the dash. When working properly they work quite nice. I've had mine defrost the glass on cold days and heats the cab up in about 5 minutes while diving, since the cabs are not that big.
Hope this helps.
