Sorry I haven't checked in here for a while.
That fender is from a VF500F. It's rivoted to an aluminum bracket which bolted to lugs cast into the original fork legs and acted as a fork brace.
I machined the 550 forklegs (longer, later model legs)round and made a clamp for each leg that the fender fastens to and again get the benefit of a built in fork brace. This fender is originally for a 16" wheel, but I laced up an 18" for this one, and it fits well. I did try it on a 19" front & it looked goofy....too small.