In the US, there are three seat pan styles (maybe 4 if the CB500 K0) is considered).
Cb500 72/73, Cb550 K0 through K2 used the same seat pan with small, minor variations in the seat cover pattern.
The Cb550 K3 has an elongated seat pan and accommodates the larger tank also used on these models. Seat cover is very different than other K models, as is the grab bar.
The Cb550F model has a seat pan to accommodate a larger tank, and the rear has a turn up and holds the tool kit under it. The seat cover is different than all the above, as is the grab bar.
There is now and has never been a CB500F model. And, certainly not an F model with a K designation. The side badge read 500 Four, which seems to confuse many.
Little wonder you are confused by the ebay advert. They are, too.
Their pictures don't give the same profile angles among the samples needed to choose the correct cover for your seat.
But my guess is that they don't really have a cover for the K3 seat pan and are simply trying to show differentiation in the impressed stamping of the cover.
Cheers,
