Steve,
My French '66 230SL has the same fuse box. When I first joined the group (well, actually, the old Yahoo! group), I asked what it was for; the best guess was radio, but I quickly demonstrated that that was not so (removed the fuse, radio still worked).
Being French, my car had a strange wiring quirk: The fog lights would come on with the parking lights, but not with the headlights. I finally traced this down to a circuit from the parking lamp connection on the light switch (#57), through that fuse to the fog lamp pull connection back on the light switch. I removed that circuit and wired up the fog lamps as shown in the 230SL wiring diagram (from the fuse #8 rather than the light switch), and everything worked as advertised. That's why I wondered about your car.
-David Pease
'66 French 230SL