Wladzislaw, thank you for your very comprehensive comment.
I just got in touch with Jean-Louis Prat, but I do not think this is the authentication of lithographs of his work.
Maybe he'll give me the contact info the next member of Chagall fund.
Added after 20 minutes
I've found this useful information here:
http://www.fineartregistry.com/artic...ll-swindle.php