Tony Diaz has a couple of these. They make regular appearances at KansasFest. As mentioned above, none of the Mark Twains are 100% functional, and they all operate just a little differently from the each other. Joe Kohn had one (not sure where that ended up.) - there are a small number floating around, though. Also, the IIsi =/= the Mark Twain. It's not entirely clear that the IIsi existed as an actual functioning machine, though the fact that it got as far as tooling before the Contessa project was cancelled seems to suggest it may have been further along than just a block of wood.