Quote Originally Posted by Jan Paalman
How would this extender work by the way? Any idea? A movable rear-element/group? Since this "extender" will not add anymore glass than it already has, chances are high that it will remain a very high quality 560mm lens, right? Perhaps even similar than at 400mm?

It does 'add glass' just not to the back end. If you look at a larger image of the lens, you can see a 'bump' behind the lug for the lens carrying strap, right under the switch for the internal extender. That switch appears to be a small lever, which I presume swings the lens elements for the internal 1.4x extender out of the optical path and into that 'bump' when the extender is 'off'.