When I disassembled the front of my '69 RS back to the firewall, the heater core got pulled and changed. In my opinion, you do not want to go that far and leave a 39 year old heater core.
Although I replaced it, I did keep my original. It is now hanging on the wall in the garage. It is clearly stamped Harrison and A 69 (January 1969) on one end. No way the Harrison name or date code can be seen. Only visible item are the in and out tubes penetrating the firewall.