How does this heater box retainer rod (J-hook) work?

Another member of FABO sent me a heater box retainer rod (J-hook) for my 69 Dart. You can see the J-hook at the bottom of this picture. I am not sure exactly how this attaches to the car to hold the passenger side fresh air box and heater box up to the cowl. It does not have a threaded end to seemingly tighten up. I assume it must just twist onto the cowl somehow. Based on the way the bottom of the box is made, the larger flat hooked in pulls up from the underside of the box. Any pictures that show how it works or an explanation would be greatly appreciated.