On the E class (86-95), there's a round-head valet key besides the rectangular-head master key.

One locks the trunk with the master key (at the trunk key-hole), turning the key all the way to the right then retrieve the key in that position (or the key comes out 'horizontally'). The trunk is now locked that can only be opened with the master key. In other words, the valet key can only open the doors and work with the ignition, but it cannot unlock the trunk.

Not sure if that's the same way your S420 is set up though !