The old style covers used only two holes - one for the PCV and the other for a combination oil filler / breather cap. The newer style uses a filler cap that does not breath, so there are two other holes - one in the back for the PCV, and the other in the front for a breather tube that is connected to a small filter element in the air filter housing.
The front hole can be on the top or on the side depending on the original application. Cars generally were on the side and trucks on the top. It should have a baffle on the inside of the hole to keep oil from splattering on the tube mouth.
If you don't want to use it, then you can devise a plug for it. If that's the way you hook it up, then you have to use a breather type oil filler cap. If you use the hole for a breather system, you should make sure to use a hose that can stand up to oil vapor. Don't use heater hose, as it will degrade, swell up, and fall off the tube connections.
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David
newbie #4153
