there's no official pictures, magazine articles, parts books, that show an alternate two strut fender part # for '75, '76 550F USA models. The new '76 F I purchased in '76 and the current original, mint '75 F all have the one strut. The center code is "390" , which is first year '75 F.
CMS shows a two strut fender alongside the single strut for some if not all of non-U.S. F bikes.