1990 Corvette valve covers.PLEASE HELP!!!!!!

Can anyone tell me why some C4 valve covers only have a hole on one side for the PCV valve and nothing on the other? I am looking at some magnesium valve covers from a 1990 Corvette and one valve cover has the oil fill cap and the PCV fitting and the other valve cover bas nothing at all. I have seen others that have the PCV valve fitting and the oil fill fitting on the drivers side and a filtered air inlet on the passenger side. Whats the deal? I have a 385 stroker with a Victor Jr. and an 850 carb and I would like to use these 1990 centerbolt covers but I am not sure if the crankcase will breathe properly. Can anyone tell me the reason GM made some like this? Thanks for your help.[COLOR=lime]