Demo System DS-16
From Sega Retro
The Demo System DS-16 is an add-on for the Sega Mega Drive used for store kiosks. Six Mega Drive cartridges can be inserted into the unit, and then switched between via an extra button (which would be hooked up to a kiosk cabinet). It was not made available to the general public, though some third-party companies announced similar substitutes (such as the Video Jukebox).
The DS-16 was not meant to be seen from the outside, and is therefore an ugly-but-practical unit. It connects to the Mega Drive's expansion port in a not-very-secure way, presumably because unlike the Sega Mega CD, it was unlikely to ever be moved during play.
Sega went through several kiosks during the Mega Drive years, and this one was primarily used in North America. It only accepts western-style cartridges.