I have yet to spot the difference visually but some rear brake plates lock the wheel once tightened if you get a miss match. K3 is I think the odd type over cb500 and 550f, but I am not sure. I think the difference is the spacers built in, wrong combo and the back plate can get too close and no longer turns. You might be able to relieve this with some extra washers ( I had same issue on my rebel - that fixed it on that. ) on the cb550k3 I ran I abused the 'wrong' back plate with a grindstone til it fitted but that was a very abused cheap old hack, these days these bikes have become a bit more precious.