Thanks for your replies and the book tip!
As the Seine also crosses Paris, I think it is possible that it was found somewhere in this area as Paris was already the capital in medieval ages, although the linear distance is ~100 km. It also seems that the plaque was removed from where ever it was attached forcefully...
I found an interesting similar piece on the internet:
https://www.pinterest.de/pin/365213851005646944/
http://www.la-detection.com/dp/message-2902.htm
Best regards
Andreas