There isn't anything other than the tumbler that is holding it on.

Is the ignition assembly still in the car? If so its probably just crud build up on the collar trapping it behind the rubber gasket on the ring.

Just take an adjustable wrench, vise grips, channel locks and turn it. You may want to cover it with masking tape or something if your worried about scratching it. But the tumbler is what keeps the collar from turning and being removed.

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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.