Checking this out for you I went to rockauto.com and did a parts look up for a 2003 Mercedes SLK320. Under the steering category you will find both the reservoir and O ring. The O ring (ÜRO PARTS Part # 0004661880) is 89 cents. You can find ebay sellers as well.


I found an ebay power steering pump listing with one of the pictures showing a good view of the clip. It's a U clip that slides off to one side.



http://www.ebay.com/itm/2001-MERCEDES-SLK-320-R170-POWER-STEERING-PUMP-RESERVOIR-0004600183-OEM-/121507620127?hash=item1c4a6b251f&vxp=mtr