Bob,

A little search on the Internet revealed the following:

Although it may look integral, the coolant overflow fitting at the top of the radiator can be replaced on six-cylinder E- and S-Class cars. To fix a leak, just remove the upper hose, place a 9mm box wrench over the socket, press the fitting out using a big screwdriver under the wrench, then install a new fitting.

If I understand what this is saying, the box wrench is to protect the two clips from breaking, no?