How about because all the cylinders are exposed directly to the airflow in a radial and not in other layouts as found in most Uk training aircraft, hence the lack of similar placards.
Well...lets examine air cooled engines.
Do they need airflow evenly past all cylinders to properly cool each cylinder?
Is it possible that because engines such as the little Continentals and Lycomings do not have their cylinders evenly exposed to the air flow like a radial the engine manufacturers require baffling to evenly spread out the airflow to evenly cool all cylinders?
Anyhow this will get us nowhere.