It's my understanding, that the axle bearings on the '55 and '56 are the same size. The '57 are larger therefore they won't interchange. To use a "P" case posi unit in a '55 or '56 rear housing, you will have to cut the axles 1/8". I put a '62 "P" case posi in my '56 stock rear housing and had the stock axles cut the recommended 1/8". Haven't driven the car a whole lot yet, but no noises or problems thus far. If I'm wrong about any of this, someone please correct me.Can someone show me where the o ring for the bearing and or seals go? Im not opening this up yet but im getting ready for it soon. I also understand may need to trim 1/8 inch off non posi shafts. Should I get 57 and up shafts too? Bigger bearings?