Thanks Hans! You may be right, and I'll check this.
What I have learned about German cars is that they are highly engineered - and there is a reason for the design. I probably have the loose clamp, which would explain the wiggling of the assembly - but why is the cutout hole in the dash so much larger than the ignition face? The bezel tabs are very thin - not 1/8" - so I still think I have a mystery.
If you take anything else on the dask, like a switch, the clock, or the radio, the cutout is within 1/10th of an inch, and the device just fits. Why would they not do the same for the ignition?
Do I have the right ignition assembly for an early 250SL? Was a later 250SL ingition assembly 'smaller' than an earlier one?