The air gap plate, or properly known as the RW plate, has nothing to do with the reverse flow plate. It is a secondary plate installed under the top of the firebox to aid in the reduction of heat loss from the top of the firebox. A air gap is left between the the plate and the firebox top so as to cut down on the amount of heat that can transfer between the two plates. It is a low cost alternative to an insulated firebox.
A diverter plate is a plate installed inside the cooking chamber, at the mouth of the firebox, that directs the heat from the air exiting the firebox, at that area where the air would be directly hitting the reverse flow plate before turning to a more of a horizontal flow.
In the pic below, you can see the weld marks showing the placement of the RW plate in the firebox, as well as the diverter plate in the cooking chamber, NOTE that neither are connected to the reverse flow plate.