I dug out my old Triumph TR3 hardback manual, and had a look through it, there is a diagram that indicates the same oil filler cap type of which you posted a pic of, and suggests that the cap is an air intake with arrows in the diagram indicating the airflow downward through the engine, and out through the breather pipe in the block, which Arwyn describes in his post..
I guess if you have high crank case pressure for whatever reason, if you remove the filler cap while the engine is running, you may feel some degree of airflow exiting from the oil filler tube..
