This is a view of the Lynndale firebox before the insulated cabinet is installed.

The top arrow shows the mark that is created when the baffle separating the primary and secondary combustion chambers is welded in place.
The middle arrow indicated the round pipe that transfers fresh air from the front of the unit into the secondary combustion chamber.
The bottom arrow indicates the location of the cast iron louvers that provide the primary combustion air to the system.

The area below the cast iron louver location is for ash removal via the ash pit door.
Cast iron shaker grates allow for easy flow of ashes into the removal tray.

This view of the firebox of a residential model during construction (before the baffle is welded in place) shows the holes in the back of the box that bring fresh air into the secondary combustion chamber.

Note also the notches for the cast iron shaker grates and the open that allows easy removal of the ashes.