It was a 464plus motherboard, with an extra "daughter board) allowing the insertion of 12 games cartridges, plus an extra special cartridge acting as an OS.
http<gallery caption="Some photos of Amstrad Expo 90">Image://amstrad.cpc.free.fr/amstrad/images/csdscreen1.jpghttpImage://amstradcsdscreen2.cpcjpgImage:csdface.freejpgImage:csddaughter.frjpg</amstrad/images/csdscreen2.jpggallery>
http://amstrad.cpc.free.fr/amstrad/images/csdface.jpg
http://amstrad.cpc.free.fr/amstrad/images/csddaughter.jpg
Courtesy of Amstrad.cpc.free.fr
[http://amstrad.cpc.free.fr/article.php?sid=16 Link to the French article on the matter]
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