From your description I think you need the castor arm support bushing. I think the part number is 126 333 54 14 or 126 333 33 14. (At least those are the part numbers for the parts for my 420SEL). Please confirm this before you buy anything as I am not 100% certain.

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Jason Priest
1999 E430
1995 E420 - retired
1986 420SEL - retired