This is a Contax 17-35mm f/2.8 N lens which has been modified and reprogrammed to function on Canon EOS cameras. This is not simply an adapted lens-- its rear flange has been removed, the contacts changed, and the actual lens reprogrammed so that its auto-iris and autofocus work seamlessly on a Canon EOS body. Compared to this lens, the Canon 16-35mm f/2.8L II is a piece of crap. Sorry, but it's true.
The waitlist on getting one of these modified is at least SIX MONTHS and orders for this service might need to go incomplete due to scarcity of parts.