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Front or rear bolt? The rear bolt is similar but doesn’t have a hex head on it, just 2 large flats as it engages into the brake plate. Probably have the front one somewhere.
Are you sure Neil?Looking at a CB550F parts book the front bolt is a hex head, 90133-286-000 but the rear is the same as fitted to the CB500K, 90127-323-000. It is not a hex head, it has 2 large flats only and slots into a special groove in the brake plate. 393 middle number isn't a 550 number, that's 390.
Have you checked the hole in the brake plate where the head of the bolt would fit? If it’s hex shaped then that’s the right bolt, if however it’s an elongated slot you need the other type.