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Thank you for the answers! I checked and it is domed what I have in under the dial. But it might have been replaced with a wrong one, that's why it isn't working apart from being badly scratched. I will try to find it basing on the ref number then.
Thanks to all for their opinion! My disc is domed, I changed it today and installed a part #560 1500, it fits and works well, so I believe the calendar discs may be widely interchangeable among the calibres of the same diameter.