It is indeed a W210 body (E-class).

The 17mm hex is used to remove the tensioner pivot arm that the shock, spring and roller pulley are attached to. It should have a black plasctic cap covering the hex head just like the roller pulley did.

If you put your car and your location into your signature (User CP) it helps everyone here help you. For example, E200 was never available here so will be difficult for you to get engine specific help, but knowing it is a 99 E200 W210 helps as many items (e.g.: tensioners) were used on more than one engine.

__________________
Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)