Actually we are both right and both wrong. I have this annoying habit of always selecting the CB550F2 whenever I go to look up parts for the 550f, mainly because that’s the one I owned, however after your post I thought, did I misread the parts book? But no I didn’t and neither did you. The 550f and 550f1 did indeed carry on using the old 500 exhaust clamp, the 323 middle number, however that changed for the 550f2, not only did they change to the 350f part, 333 middle part number but they ditched the spring washer and went to a flanged 6mm nut.
No idea why Honda suddenly changed the clamp, maybe they ran out of the 500 clamps and were scratching around looking for an alternative. Or maybe it’s 2 grams lighter and as they say “ every little helps”