These would be the new "START, SELECT" button positions
The top version uses full size buttons like the other ABXY buttons
The bottom version uses smaller buttons because of the limited space near the stick and speaker (don't forget the speaker itself is larger than the grille pattern). These would be better when playing vertically oriented MAME games. It could also be used for vertical shooters and Arkanoid type games like we have on GP32. The current GPx2 design would be very awkward for these as you would have to crook your hand around the unit to fire/jump etc.
These would not be too hard to change, there are more elaborate schemes but would be to much to change this late in developement.
The volume could be re-located to the side as a wheel like GP32 or be placed where the start and select were under the ABXY buttons.