The Cool Chromaticover. You can’t really tell from these terrible product pages, but this is a standalone MIDI controller with velocity and several slightly different key arrangements available, that just happens to be made to fit over the keyboard on a Yamaha combo instrument. Unlike the Yamaha (or most other MIDI controllers) the Chromaticover appears to be really well-made, mostly metal with metal buttons.
The only dealer selling it (piens.be) is also the owner of the company making it (cool.be). Also strange is that very few people in the accordion world, or elsewhere, even know this product exists. I’ve found very few Google mentions of it.

(This was discovered by the diykeyboard group, who now have their own wiki.)