I've seen a fair few of these lenses changing hands online (the "cheapest" one popped on ebay from a bird photographer in Korea some years ago)
Looks like there is one here as well:
So I wonder if there are really only 20 of them existing or it is just a number to drive prices up, of course it could have been the same copy multiple times.
On the other hand, I never saw the Nikon 1200-1700mm f/5.6-8 lens popping up, all-mechanical, no electronics to go wrong, the 1200mm f/5.6L is a risky buy in that regard (just like the 50mm f/1.0L).