I got a chance today to look at the 50/51/52 flywheel fans. I took them off and held them all together and can see no difference. The OD, ID bolt pattern and thickness are all the same. The color and material appeared to be the same and I saw no markings of any kind on any of them. I would say they are fully interchangeable. They do have different mounting hardware. The 50/51 use a flat screw head and the 52 has the allen head. I didn't notice it, but there could be a slight difference in the recess where the screw sets down in and that is the only reason I could think why the part numbers are different.