I removed, the white spacer, the crown stem, the hands and the dial and had a look at the keyless works. And much to my surprise, I found the crown gear laying loose on top of the movement. Lucky that I had carried the movement carefully over to the photo table (this shot was taken after I had replaced the crown gear). I did not have much intention to sweep the living room with the magnetic sweeper looking for this tiny castle wheel. Looking at the problem, there was only one possible cause. The castle wheel (called clutch in the USA) lever did not retain the castle wheel properly. And a check of the screw inside the black circle showed that it was not tightened properly. I was very relieved, that was a very easy problem to fix: tightened the castle wheel lever screw, labeled "loose screw" in picture below, pushed the castle wheel under the castle wheel lever and placed the crown gear back in place, inserted the stem and finished.
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Picture added July 2004