Thank you for the very interesting answers.
I still believe though that there is something strange in this siebel. Please forgive me for my stubborness, but checking a higher resolution image of this photo it still seems to me that the part of the circle from the scratch downwards is separating from the red background (actually casting a little shadow). And no trace of damage, in that area.
http://images.google.gr/imgres?imgur...%3Del%26sa%3DG
thank you for your attention.