It looks like the knob is missing, but the purpose of it as it was explained to me isa "foot warmer". If you turn it to open, then look underboth of the cylinder banks at the metal covers, you will see a triangle shaped door that will be open. that is the door that the knob controls. It allows hot air to be sucked out and pass over your feet.
BUT thats just what i was told. It sounds like a bunch of hooey, and plausible at the same time. I leave mine open year round. Winter for the heat, and summer to dissapate the heat in the engine compartment. Better safe than sorry.