Greetings Metz Masters,
My understanding is that the cut out was only used when the motorist needed additional power, as when hill climbing, and thus hot exhaust gases would not often be blown onto the wooden running board support.
My '14 Model 22 has a piece of sheet metal attached to that cross member to protect it from the hot gases when the flap is open. I do not know if it was installed by the fellow who restored my car back in the late '60s / early '70s or some previous owner. I doubt that it came from the factory. I have seen a picture of another Model 22 with a piece of tin wrapped around the wooden support as a protection.
It is interesting, and as yet unexplained, that while all of the Model 22 mufflers have the cutout flap my car never had the pedal used to actuate the flap. There is no slot in the floorboard for the pedal. They are the original floorboards and not replaced.
Good luck with your projects, Bill