you will find a plastic holder with a foam filter in it to collect the sludge which is in the fumes which will rise to this area during idle or after shutdown when the PCV is not pulling air from the air filter.
The only time the PCV is not pulling air from the crankcase is when the engine is not running. The filter in the side of the breather is to prevent dust from being drawn into the engine, the same thing the horsehair packing in a vented oil filler cap did. The slotted valve cover let all kinds of dust in, and was at least partly responsible for the short piston ring and bearing life of those engines. When the "closed loop" PCV systems came about, engine life virtually doubled due to less contamination in the oil, and more effective purging of blow by and acids formed by humidity and combustion gases getting together.
Jerry
