Sega Master System / Mark III / Game Gear
The Master Gear Converter is a third-party add-on for the Game Gear that allows you to play Master System games. It was also known as the Master Gear, Gear Master Converter and Master Gear Adaptor. Some variants were officially branded Sega items.
It is a simple passthrough adaptor from the Master System cartridge to the Game Gear cartridge slot. It additionally connects a pin in the Game Gear slot to enable its built in Master System compatibility mode.