Genesis:32X Model Differences
Page content & researching taken from io55.net's Genesis Add-Ons page with permission, in collaboration.
Models are arranged chronologically. There are no revisions.
“32X MK-84000” (1994-????)
These 32Xs have some problems with Model 2 Genesis consoles of VA3 or 4 revisions. If nothing is plugged into the side expansion slot (typically used by the Sega CD), many 32X games will refuse to work. While one could plug a Sega CD in to resolve this issue, there is also a “32X Extension Unit” available, which simply filled that side slot with nothing. These may also have a narrower cartridge slot, but this needs to be verified with measurements.
“32X MK-84000A” (????-1996)
The compatibility issues with VA3 and VA4 Model 2 Genesis consoles have been fixed with this version. To identify this, just open the cart slot and see if the PCB’s code ends with “A”. Furthermore, these may have a slightly wider cartridge slot, but this needs to be verified with measurements.
https://gametrog.com/sega-32x-information-specs/ (incorrect on version differences, slightly)