In the photo of my current cart you can see the Red wires connected to pins 41 & 42 of the cart edge. These I have connected to a jumper on the outside of the cartridge enclosure.
When these two contacts are bridged or unbridged determines if the GameGear is in a SMS or GameGear mode. There are a few Cartridge adapters for the GG that allow you to use SMS cartridges.
So one could also use this flash cart to play SMS Roms.
All the more motivation to get this puppy running
-Gerry