When you look on the inside of the covers did you notice a round impression where you would normally see the hole for the breather? I just bought a new set myself and as they can be for either side there is a small spot that you can hit with a hammer to knock a round hole in it. At least thats the way the set I have were setup.

 

The set I got had nothing on the outside, I new where it should go and the pattern on the outside changed around the area. I wasn't sure what was supposed to happen but I tapped it and it just popped out. If I were you I would pull it off, as I would not want to tap the piece in, or drill and get shavings in the valve cover.