I played around with the code and a Game Boy Camera module, but it all was very weird. I was able to read the header once, and only after I unsucessfully read parts of RAM I was able to read the header again. I'm testing cartridges with RAM modules and different memory bank controllers. I should write down everything I learned about the Game Boy at some point.

Game Title: METROID2

Cartridge Type: 3 - MBC1+RAM+BATTERY

ROM Size: 3 - 256KByte (16 banks)

RAM Size: 2 - 8 Kbytes (1 Bank)