Hello, Mario, have you checked Zinner's book 'Alte Sonnenuhren an Europaeischen Gebaeuden'? Ernst Zinner, german astronomer, has compiled this book of several thousand sundial sites including many in Italy. This book was released in 1964 and contains only sundials before 1850 (of course medieval sundials are older). It was the basis for the catalog of the sundials (8500) of Germany and Switzerland which was published by the Arbeitskreis Sonnenuhren in 1994.
An expert on medieval sundials is Herbert Rau from Berlin. If you don't know him, I can give you his address. I'm sure that he can help you. - Daniel Roth, Arbeitskreis Sonnenuhren, Germany
