As far as I know, the Mamiya Universal, though both are very similar to your Polaroid 600, they are also totally incompatible with each other.
Thanks! I've been looking, including on the MF stuff. But, to no avail. The lens mount is different of course (I have both), and the back tangs are slightly different, with some SE backs fitting Universals 100%, and you could also go the other way if you notched the Universal tang SE-style.
But, my issue is away from this, it is if or not the 545 goes into an `M` adapter, a `G` adapter, or some other Mamiya thing. I had thought the 545 would fit into the `G`, but it appears not (as many of the Mamiya back parts are also available with SE mounts)... Oh the pain. Skj.