I'm betting it's a lower manual that will sit/attach under the Tyros, allowing for dual manual control....maybe becoming a stand for it, and allowing bass pedals.
There seems to be a lot of interest in dual manuals, especially on Jazzhooves, who are "organ" enthusiasts.
Ian
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Yamaha Tyros4, Yamaha MS-60S Powered Monitors(2), Yamaha CS-01, Yamaha TQ-5, Yamaha PSR-S775.