Well from going through auction databases I only found 6 ever sold at auction. Through friends I know of 2/3 more in private hands including mine. That would make for a total of approximately 10 pieces known. So if we want to round it up we can say there were maximum 20 ? I don't know this is just my estimation, I may be doing it completely wrong, let me know what you think !
The calendar is in Italian
Cheers !