Inside the cartridge. Note, it's 256K total. The address space for this unit was only 32K for the cart port. There has to be some banking scheme going on here to support all 256K. Also, note a power connector is on the board.
Note, the last vendor to sell accessories, High-Tech Solutions, dropped their website a long time ago. Here's a historical list of what they sold as far as accessories: http://web.archive.o...Accessories.htm