from what I understand the sega/de roms going a significant version number like 1.x to 2.x etc was either a change in the 'OS' or change in the code that ran the DMD. The DMD rom must match the cpu rom, eg: both must be 1.x, you cant have a 1.x cpu rom and a 2.x display rom.
Stern itself only lists the 1.3x rom, so the 3.x rom might just be a hack. could just be a few bytes swapped for a scoring change or a slightly different switch test. (oops I see that stated above).