Im curious how big the inside "chamber" of the upper area is and, if there's a chimney (so smoke flows through the chamber) though, I don't think I see a chimney. Does the grill area go underneath the "chamber"?
I'd light the grill up with a thermometer inside the chamber to see how hot it gets inside. I'd assume it was used for bread as the "mailbox hole" isn't very big.
Heck, if the floor of the chamber is big enough (and flat) and gets hot enough, I'd be tempted to open the mailbox hole wider to make a pizza oven.