If it has a badge attached to the cover, it is a K model, as the F side covers had vinyl letter applied.
The 74-76 cover badges had a plastic orange "F" embedded into the molded badge itself.
78 CB550K badges were molded and gold in color with black paint in the shadow of the letters.
Not sure I've seen 77 badges, if they are different from the 78 models.
Picture : top 75 CB550F, Left 74 CB550K, Right 78 CB550K)