I have an F1 and had trouble getting a fork stanchion for this model. Strangely cruzinimage did not list them on ebay.uk and bought 1off from Germany. Only found the cruzinimage ones by searching ebay Australia!
There are listed three different fork tubes for the CB550:
K1, K2: 51410374003
F1: 51410390003
K3, K4: 51410404003
The differences are to do with the oil damping holes and some forks have a rebound spring and some not, but I never did find all the details.
This pic from an ebay listing in the US gave some detail of parts that the F had over the K1.