A manual was not available, only on the box was a Polish text instructions. No further translation or images.
After I had the holders mounted on the wall, I wanted to push the shower rod, but realized that the it does not work - so I unscrewed the brackets again and tried to push them over the shower rod first. Unfortunately, none that bore through the brackets is so tight that I could only push a few centimeters over it the shower rod, around the bend, it should not go. The lower end is provided with a non-removable holder for the shower hose.
Faszit: the thing goes back. So, as the box looked, I was probably not the first person who has dealt with this fitting. Why that should be a top seller, is beyond me.