Can't get to my F easily as it's tucked up one side of the garage, but did manage to measure. The foot lever arm on the 500K is about 21cm out from the left leg and on the F it's only 16cm.
As far as I know there are just the three stands for the 500/550s. The 500s and early 550s (US) models with the long arm, the F version with the short arm (both had a tube connecting the feet part way down)and the 550K3 which is a different construction (no tube connecting feet), but also has a long arm. If you still need pics let me know.
Phil
Correction: another type was the F2 stand which is similar to K3 construction, but with short foot lever, so four variations in all.