What is this nut?
 

I backed my 2001 SLK230 out of the garage and this nut was on the floor about in the middle right side of the car. It has coarse threads and 3 indents to lock it in place, I guess it didn't work. The back side is recessed as if to fit a washer or something. I'll be jacking the car up tomorrow to take a look.
Maybe rear suspension, exhaust, etc.

Thanks.

I'd check the threads to see if it is metric or not. ( a 3/8" coarse will sort of thread into a 10 MM nut especially a thin one so be careful not to miss ID the threads )

The back side looks like it was spinning around for a while and got dirty except for a very slight portion.

I'd look at the lawn mower / tractor especially if non metric

Uh... That holds the passenger side smoke shifter in place.
Its easy to put back on but you will need a pipe stretcher to get to it. ;p:vbac47679

Since you mention the lawn mower, I pulled the SLK out to drive the Snapper rear engine rider into the garage. I need to check the thread size. Maybe it came off of the new Snapper rider.

Have a close look at the nut to see it it has ever been installed. ( marks on the corners , threads on the self locking area rubbed / deformed.

Given the mower is new, I'm suspecting it was a stray nut that never got installed and has been sitting on a flat surface vibrating along. Look for an area where paint is rubbed.

Also look for the same nut on the machine.

Problem solved!

Thanks 97, upon further inspection, it turned out that the nut came off of my 25 year old Mclane edger. I was watching TV last night and a vision came to me.....I had seen that nut the last time I waxed my lawn equipment, and the rear wheels on my edger came to mind. A quick trip out to the garage and sure enough, the nut on one side of the rear axle was missing. I was lucky it fell off when I was going into the garage and not out in the yard.

The recessed pattern on the back of the nut was caused by the steel wheel spacer rubbing against the nut. Now I can sleep!;)

I still think its a rauchschieber

You're welcome. The nut wasn't looking very modern MB. Having been in and around the auto repair business for 40 + years coupled with 20 in industrial machinery repair, I've become a pretty good detective.