Wish someone had told me about that key and how it operates a day or so ago. I am sitting here with bloody knucles after just completing changing the belt without removing the fan. The most difficult part was getting the belt between the fan hub and the crankshaft pully. Also I thought the 13 mm bolt was on the face of the engine near the 19 mm bolt rather than at the front top of the engine. After rereading your original instructions I realized where it was. I spent two days looking right past it!
Oh well... water over the dam.
Now to change my master cylinder and left rear caliper.
1993 190E 2.3
2000 Toyota 4x4 Tundra