This kit contains a replacement 28-pin BIOS EPROM and EPROM socket for the original Sega Master System (not the redesigned SMS2), PCB revision 837-6067.
Installation requires a small PCB modification, detailed in the wiki
This BIOS bypasses region checks and allows you to choose which port to boot SMS software from, Cartridge, Card, or Expansion.
This BIOS is compatible with 837-6067 and 837-6405 revision PCBs. These are SMS units with no built-in game, leaf switches for the Pause & Reset (6067), and a few other hardware differences. It may work on 837-6629 but it is not guaranteed. We had to alter Bock's code a bit to get the Boot Loader to work on these earlier PCBs.
It has the advantage of not checking any country code or header, so basically it is possible to boot up an original Japanese mycard game (for SG-1000 or Mark III) on your US or European Master System 1.
You can also use it to boot Brasilian 8 megabits games which are not supported by all models of Master System 1.
Last but not least, using a homemade pinout converter, it would be possible the same way to boot up any SG-1000/SC-3000 or Mark III/SMS cartridge.
For information about the SMS Boot Loader, see the entry at SMS Power