If you are in a bind, need a window stop bumper screw and can't wait for shipping from a Mercedes supplier, or don't want to pay for shipping, try a "Hinge-mounted Door Stop" from the local hardware store. Take an M6 nut with you to check the thread size and pitch. They are almost always M6 or 1/4" thread. You will need an M6 thread bumper screw
Here's the one I bought at Canac Hardware here in Quebec. The same company had different models, some with 1/4" and others with M6 threads. The one shown is M6 and worked fine. It's the wrong color and it's nylon but it's inside the door and not visible. If that's a problem for you, you can always use the hood stop screw in the door and order a new replacement M-B part for the hood stop screw. The M-B part numbers for the correct window stop and the hood stop are the same and the hardware store model will work for the hood temporarily so you can permanently replace the window stop with the correct M-B part and close up the door.
Tom Kizer
Levis, Quebec, Canada