Very are and very desirable.
There exists perhaps 10 known examples, all in private hands. We do not know where they all are, or if that estimate is valid. I suppose an upper ceiling of 20 examples may be valid until we see otherwise.
Here is one I recently acquired. It shows typical signs of ownership: The chrono hand is unlikely to be original, as with the blue hour subdial hand. However I think I shall keep all as they are. These are so rare, that each may be identified by its small deviations. So the hands are part of the history.