The dipstick looks too short?

 

It certainly looks too short. Maybe it is for a model that just has a plug and doesn't have the oil filler pipe? FCPEuro doesn't list any fitment for it. It could be a mistake. They also sell a transmission dipstick that has the same level markings at the bottom, but it specifically does list the SLK320 under fitment. I have used one of those for my ML320 and plan to use it for the XF. I have used the red locking tabs as well, but I also bought a new cover.

 

What reading should the dipstick say when you know you have enough?

 

The correct dip stick has two marks, one for when the trans is cold and the other when it is hot. And remember, you need to measure, and the car has to be running.

eBay has them.