Ah, Mercedes part number. Search ebay or google using part number 1708900314. You can get a brand new one for about $15 - $25. Says it's a spare tire mount, like the post above identified.
If the bolt looks like it was professionally installed and is safe, you can find used Emergency spare tires (spacesavers) for SLK230/SLK320 on ebay, if you wanted to carry a spare. If it was covered with a tire cover, I guess it wouldn't be so bad to have a spare back there.