I am inclined to think that there is no appreciable difference except, perhaps, a difference in age/manufacturing dates. That might introduce some variable, but not enough to matter. I think that if one were a circular polarizer, it would be marked so; they usually are.
BTW, I love my 50. It is a bit clunky, but it is sharp and renders very well, indeed. Maybe I should get a polarizer; right now I only use a KR1.5. I had trouble getting used to using that extra focus ring, but once I got the hang of it by reading Hasselblad's official instructions, it seems easy.