It's Vemia auction time again (the private eBay for Vintage Synths And Cool Gear). If you can battle through the javascript errors to this page, you'll find a Yamaha CS5 synth, currently at £140: "This was superficially just a nice-sounding early Yamaha analogue monosynth...Then we turned it over and saw the beautifully-done inscription for an Aphex Twin record release... A little more research, and we found that these were the sleeve notes Richard inscribed and then photographed for one of his major releases ever, if not his biggest ever release: Selected Ambient Works Volume II. As well as the main inscription, there is... a false start on the main inscription, in the gulley at the back right of the bottom of the synth, which reads 'Written and prod'. See the big image here. Seems that Richard James originally asked them to sell the synth anonymously because he'd forgotten about the inscription. Other wonderful lots include: this, this, and the inevitable Synton Syrinx, Yamaha CS80, and VCS3.
UPDATE: Yes, as many people have said, Vemia is served by the Worst Website Ever. It seems to work if you click on the home page first, then on the specific links above. But it might not.