Go to http://www.progettoemma.net/ Than go to ADVANCED SEARCH After that You should select *mature* under CATEGORY all the way from Misc *Mature* to Tabletop/ Othello *Mature*. You should do that manually because site's front end does not allow selecting only the word MATURE alone ??! But You are willing to do this operation manually as U said, so that is the solution for You. (and others, too)
P.S. Can someone ask on that site for implementing this particular search option ?