They don't need to be opened up necessarily.
If the beeper pitch is the same as Mephisto I/II, then it is 3.57MHz. If it is a couple of notes higher pitch, then it is 4.2.

Higher CPU frequency in this case doesn't mean a stronger program. The IRQ is faster too, which means the internal chess clock timer is also faster. So, it will respond a bit faster, but will calculate the same amount of possible moves.

It is also possible that the 3.57MHz version is an older program revision (different ROM)