Thunder and Plain. I think you will see that an "Air Duct" as defined in the rule book is allowed in Gas Coupe. For the last 35 years I have been the association the were not a body modification. Bruce Gieslers "Hanky Panky" #219 53 Studebaker has one and runs in gas coupe and has for over 40 years. I almost put one in my 1940 Chevrolet after I ran over 150 and the rear became very light.
If you believe they aren't I would contact the head of the Coupe and Sedan committee chairman. I would also use this car as an example. I could be proven wrong on this but with Bruces car as an example I believe I'm correct. It is always best to call these Committee people that's what they are for. If they are not legal I would not want to be the one to tell Bruce.
These devices are not as popluar as they once were and that maybe because of the more aero-dynamic bodies available and the running of cars in Altered which allows the spoilers. . . . JD