If its anything like the covers we are using for the NW dual CAI that cutout is for an oxygen sensor (its the MAF sensor not an oxy sensor, thanks James). When I put it on my car one of the things I noticed was a large hole that looked like some type of breather or oil fill cap went there. I asked a Mercedes dealer what went there and he showed me the blow up on the parts screen and it was the sensor with no cover over it. I got a piece of plastic pipe at Home Depot and a cap that fits it, painted it black like the engine cover and stuck it in the hole. Others have used wheel center caps etc. When anyone ask what it is I just make up stuff, "its where the nitro methane goes".