I've searched various forums and practiced depressing the woodruff key on my new cylinder with a clothes pin which also seems to fit the cylinder currently on the car. The problem is that I'm not feeling the cylinder release forward and I can't turn the plastic barrel that surrounds the cylinder. Do I need to use needle nosed pliers on the barrel after I've depressed the woodruff key with the clothes pin? Thanks

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'81 300SD

Not sure if the w123 is the same, but what I did was use a paper clip (thinned out on the end). I believe the key needs to be in position1 before you push the pin in. Once pushed in, you should be able to spin the barrel. In the w123, i found it easiest to remove the actual metal part of the key from the plastic key handle. Again, I did this on a w123 and not a w126. I did find this.
W126 Ignition tumbler replacement « Classic Jalopy

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1985 300d Turbo Diesel

Thanks for the link. I figured that pliers were going to be involved at some point.

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'81 300SD

The collar came off easy on my SD that's missing the rubber surround. It was difficult to get a good enough hold on my other SD (84 & 85). You also need to make sure that the locking tab retracts enough to let the collar rotate. Perhaps jiggle it to release with the paper clip inserted. Experiment on the new part so you see exactly what has to be done.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.