BackBit Firmware 3.4.2 (Legacy) - Sadly, this appears to be the last firmware available for the original BackBit hardware, and the previous firmware versions are no longer available from the official firmware source.  Additionally, there is no mention in the revisions.txt file of what the changes are for this version.  Assuming it's feature parity with the BackBit Pro up to this version, then the changes would be:

• Version 3.4.2 - Sega Master System: Joystick debounce fix