mk2/r395 scubapro

Oops I just impulse bought this off a woman who never used it thinking maybe I could add the second stage to my MK 20 or 25 (need to check)/S500 and ditch the AIR2 BC so I can try a BP/W as so many here recommend. But now I’m confused by the diaphragm, piston, balanced, unbalanced stuff. Can...