The bottom box in the upright is a difuser box. It has holes in it to manage the heat from the firebox. The holes get bigger from right to left. It is similar to the convection plates sold by Horizon Smokers except for the shape. Yes, it took three of us to get it off the truck.
OK Joe's Longhorn & Vertical Roasting Oven, Brinkmann Cimmaron,'69,'73,'84,'89 Weber Kettle