If I remember correctly, the Fox motors that were considered competition grade or Combat Specials either had a name attached or an "X" added to the engine size. I think the order of development was the 36X, then the Rocket, then the series of Combat Specials. The last one was the Combat Special Mark VII. The last one was developed to compete with the Nelson's but couldn't quite hack it. I believe the ABC versions started with the Mark VI (or maybe the IV, but I can't say since I didn't have any IV's).

The earliest competition motors had round backplates and square venturi (36X and Rocket). The series of motors that were actually called Combat Specials and CS Mark "X " had the square venturi and the barndoor backplate (to make Schnuerle manufacturing easier). This case design was also used for sport and competition 40"s.

I flew competition C/L Combat for almost 20 years. Started out with a Rocket, and finished with VI's. Could fly all day on 25% fuel. Would sometimes use 45% at a contest, but depending on the weather 0might have to use a thicker head gasket.