Before you buy the tumbler I would remove the one you have. My wifes 300D had a similar problem and it was not the tumbler but part of the ignition assembly.

Take a look at this page I am working on..

http://garnet.acns.fsu.edu/~jjs5772/W123

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'84 300CD Turbo 132k (Anthracite Grey) - WVO - My daily driver - Recently named coo-coo-coupe by my daughter.
'84 300D Turbo 240k (Anthracite Grey) - Garage Queen
'83 300D Turbo 220k (Orient Red) - WVO - Wifes daily driver

I'm not a certified mechanic, but I did stay at a HolidayInn Express last night.