I did some tests using Acetone with some other old gaskets, but even after bathing the parts a few days in the solvent had no helpful effect. So I decided to go ahead and give it another try to get the gasket out mechanically. And I succeeded.
I could even do some measuring. The gasket in the old Narrow Flat Feet crown is a single zylindric one. Compared, the new crowns come along with two staked round donut gaskets. The inner diameter of 2.1mm seems to be identical. And also the width of 0.8mm. But the height differs. Whereas 2x staked donut gaskets have a total height of 1.6mm, the zylindric one has around 1.2mm. And the 1.2mm is approximately actually the available space in the crown.
I couldn't find a zylindric gasket. So I have to decide if I want to give it a try with two staked donut gaskets that are usually used for the new crowns. I'm not sure if the two gaskets could be compressed by around 0.4mm in height.
Anyhow, here are some photos as reference for the community:
View attachment 1568433
View attachment 1568432
And this is the "donut style" gasket that I've used on the newer crowns. Two of these staked:
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