Backwards Compatibility Game Cartridge Adapter - Dismantled for reference and curiosity
A selling point of the Mega Drive is that it's electronically backwards compatible with its predecessor the SEGA Master System.
Physically though the ROM cartridge ports are very different so there's was no way to play a Master System game on the Mega Drive out of the box. Enter the 'SEGA Mega Drive Master System Converter'; known as the 'SEGA Genesis Power Base Converter' in the United States.
Once connected to the Mega Drive the Master System Converter allows users to insert Master System cartridges into a port under the top cover as well as run Master System card based titles through the forward facing slot. Like the original Master System (not the Master System II) the card port supports use of SEGA Master System 3D glasses. The converter also replicates the Master System's 'Pause' button not natively part of the Mega Drive.
The Mega Drive incorporates the Master System's Zilog Z80 chip as a coprocessor and when a Master System game is run the Mega Drive switches to the Z80 and puts its 68000 processor into an idle state. While the Master System Converter does have some components and is more than just a pin converter, it does not contain any specific Master System smarts and is in effect a passive, hand holding adapter.
The Master System Converter pictured below was designed for the original Mega Drive shell and is not compatible with the Mega Drive II. A redesigned version was created for the Mega Drive II that it didn't include the game card port, was only released in Europe and in limited numbers.