Thanks for all the responses!
I can tell you that it is not the starter bolt access plate. That is up higher and to the left on the trans hump. That plate also allows access to the well hidden trans solenoid. The a/c hose holes are much higher up, but maybe it may something to do with the evaporator or fan? It is such a large plate that it has to have some purpose. I might tend to agree with Matti and the idea of a carpet form. There is no hole on the backside that I can find either. On my car, it is opposite the heat shield for the exhaust heading downward. I have had the shield off and there is no hole.
Maybe one the great Mercedes mysteries.....