The "What is this hole on the air filter cover" game
 

The car is a 1988 300SE. While hunting around the Klima, I noticed a hole almost in the middle of the air filter cover. It's just closer to the windshield than the Mercedes star emblem. The hole has a rubber grommet inset into the hole. If I didn't know better, it looks like the hole old American cars used to put a wing nut on to hold the filter closed. On the MB I have one nut and each if the 4 corners. Then there's a hole with a black rubber hose attached to it. Then there's this hole I'm speaking of that has nothing and is simply a hole with the black rubber grommet.

I did start the car and plug and unplug the hole. I didn't notice any change in the engine.

Thanks to anyone who can tell me what is supposed to be in this hole.

It allows you to adjust the mixture setting of the fuel injection
system. That said, I have read that you just don't bother.
Anything you do to adjust will be overriden by the computer.
It is only for emergency situations like when nothing else is
working anyway.
Am I first? What do I win?

Am I first? Do I win? My friend, you were the ONLY one to reply. I was worried that the hole would allow unfiltered air to enter the engine. Do you know if there's supposed to be some sort of plug in the hole? I have nothing for you to win, but I can say that I did my part to help the Canadian economy this year. My 2 brothers and I came up for the Canadian GP. Good event, I just wish the track were better labeled with signs for walking around.

My air filter housings were just like yours with no plug. I can't
imagine any unfiltered air getting through the hole to the
engine. Next time I'm under the hood, I'll look to see how
they blocked it. I suspect that the rubber grommet isolates the
mixture screw from the intake air.

Extended grommet
 

Take your air cleaner cover off and you will see that the underside of the grommet is extended to mate with the metal housing around the adjustment screw. It is this seal that keeps unfiltered air out of the intake.