OK. Mine has a steel sleeve over the spring, but it is not retained by any nut. Once the lid is off, the sleeve and spring simply pull up, then the steady plate etc. I cannot say if the 300E should have a nut retaining everythin inside. perhaps one of the E owners can chime in.
Have you owned the car since new? If not perhaps the PO added a little something extra to hold the innerds in place. Can you push down on the sleeve enough to see the unthreaded portion of the rod below the nut? If so, try to gently get on that portion with a small set of vise grips to hold the rod steady while removing the nut.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.