The O-ring,15x2.5 is the lowest priced item on this Gasket Kit A schematic.
Parts such as the Gasket Kit A: O-ring,15x2.5, Gasket,ex.pipe (nas), Gasket,breather P (mca), Gasket Kit A (mca) are included in the Cbr600f Hurricane 1989 (k) Italy / Kph Gasket Kit A parts fiche.
In this schematic the "Seal,valve Stem" is used most often.