There were 14 listed in a register published in the CPMS Newsletter last June (page 42). Yours would make 15 if it isn't already listed.
It must be recognized that note registers are works in progress. It would be altogether too presumptuous to declare that only 14 -- or 15 -- exist in all the world. That said, the numbers do make it clear that the note is a tough one; there is no doubt about that.
