The pictures about me finding these chairs I posted
here.
They had at total of six of those spintop chairs in the youth hostel in Würzburg.
I tried to use the area where you are supposed to sit as an arena for a throw top while the chair itself is spinning: It did not work very well. The curvature is to steep, the surface too rough, the hole in the center too big.
It would in principle be easy to make a better spintop of that chair by improving its tip. One would just need to place it on a ball about the size of a tennis ball - I did not have anything like that with me.
Or one could place it on top of a larger spintop then I had with me - maybe I am better equipped next time I come across one of those spintop chairs.