The part shown in the post above isn't what you're looking for. However, a Google search of the part number will bring up the bolt you're looking for. I just haven't been able to find the standard bolt size to use instead of the golden bolts Mercedes supplies. If you give me/us the VIN number of your car, we can do a search for your particular vehicle and see if it belongs there.
2014 EV Coupe 17,500 miles
2014 EV Cabriolet 9,500 miles