Thanks to all the threads on this forum warning about needing to turn the key to position 1 in order to easily replace the tumbler, I purchased a replacement tumbler with key. Luckily so because it took a lot of jiggling to get my key to position 1 where it now remains until I can figure out how I get the black plastic cap off.
I can see on the new tumbler the spring loaded gizmo that needs to be depressed to remove the tumbler, but the black plastic cap needs to be unscrewed I think, and it's resisting being turned. I'm thinking that I need to depress the spring gizmo on the tumbler to unscrew the black plastic cap but so far that hasn't worked with my homemade paper clip tool.

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1985 300D Turbo
"Evolution is God's way of giving upgrades" Francis Collins