Dear Blackstridaaustria,
how do you say in German? “Einen hab’ ich noch!”
I found a possible quick and dirty solution for my problem: An stainless steel M8 lockwasher perfectly fits into the hole of the cup spring and is with a thickness of 1,7mm very close to the material thickness of the cup spring. With the lockwasher I am practically able to “elongate” the axle stump.
But of course your recessed washer is a much more sophisticated solution.
Sorry for not replying your private message what I will do now. I’m simply not familiar with the forum functions.