rb20, rb25 s1 and rb25 s2 are the same phyiscal AFM
the difference is colour label and rb25 S2 has 1 less wire (shared ground).
to use rb20 or rb25 S1 in S2 you need to duplicate up the ground signal.
the rest is the same.
rb20 - 4 wire - ground, power, signal ground, signal
rb25 S1 - 4 wire - ground, power, signal ground, signal
rb25 S2 - 3 wire - ground, power, signal
check my site for pinouts etc
check howto section for a thread cubes did on wiring and changing them