What is it?

I believe it's the JAMMA board that takes MegaDrive carts. We had the machines in the UK, called MegaPlay 10 or something. Rather than having 'credits' as such, the games were timed and you had to keep putting money in to stop the timer running out.
Well...while I've never actually seen this unit, I'm pretty sure I know exactly what this unit is. All this unit does is allow you to swap between several Mega Drive games by simply pressing the game switch button while using one Mega Drive system cartridge slot. Instead of having to manually swap games, you can just press the button to select a new game to play. It was most likely intended to be used for store retailers who had a Mega Drive system up on display, which is probably locked up behind some casing to prevent damage or theft to the system. With that in mind, retailers would prefer to use this switch in order to allow potential buyers to test out the system or a game that they would consider buying without having to unlock the case to the system and swap out games depending on what game the potential customer might want to demo.

All in all, this looks to be a simple multi-cart switcher. Nothing worth buying in my opinion, unless your too lazy to swap out a cart from your system.

Hope that helps answer your question....further more, I hope I'm right!