I've got two mounting plates that fit onto a Polaroid CB103 back (just your normal pack film back). The one has the two pronged teeth (pictured) to fit a Polaroid 600SE, the other (not pictureed) has no teeth so it will need them to fit either a 600SE or Mamiya Universal. These seem to be at least moderately rare so they're priced accordingly.
Looking to get $40 (together) + S/H from Ohio. If that sounds steep, message me and make me an offer.