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

After you bolt the case to the firewall, open the right side fresh air door. Install the rod through the opening. The hook end goes up and hooks on the edge of the round cowl opening. Before you install the box look at the bottom of the right side below that right fresh air door. Locate the hole in the bottom of the box. That will tell you where the rod must be located. If your rod is flat on the bottom with a threaded hole then a screw goes up from the bottom of the box on the outside and through the hole in the rod to secure it. It takes less time to do it than it took me to type this.