The KW/Challenge mixer has to be the winner it was basicaly the the w900 kit with different wheels and a mixer body. It's also the reason for the demise of the torsion bar AMT switched to the Hendrickson suspension for the mixer kit. Having only been issued once and the w900 molds probably gone it's likely gone forever. As far as versions of kits go the tournement of thrills C ford and auto hauler is certainly quite rare.