Yup, definetly two different designs and yup #3 is definetly what you should be twisting if you had an MB knob.

Stupid me, you don't. you have a "Sport". Tell me if I'm wrong, but I think you have an AMG Sport specific knob. At any rate, the shaft design is the same from your car to mine. I know how the MB knob locks onto the shaft and I know that #3 is the lock (although the seperation between the knob and the collar may be between #1 and #2 this would not be my first guess). Try putting a standard screwdriver blade between #2 and #3 and twisting. Do it as far in back as you can with the car in "Low 1" so if the surface gets marred a little bit it won't be so noticeable. The "twist" is not nessecary to execute the lock but the "snap" is. I'm hoping you can get the collar to snap down the shaft. Please post your results. Paul