Hey stevo, yes, I can't find any evidence anywhere that there is any such thing as a 1974 CB550F supersport, but the frame I bought off ebay had a compliance plate dating it as manufactured in 1974! The Californian number plate that came with it had a rego tag which expired in june '95.
Any way I think the '75 ss fork ears would be as correct as I can get - though possibly it originally had the smaller blinkers - I've already decided to stick with the bigger lollipop blinkers as the grab rail I got for it is for the bigger blinkers. Looks like I've gone and picked the rarest type of fork ears!
Edit: maybe a 12/'74 manufactured is classed as a 1975- I didn't originally notice that it was december 74!