See the link below for information on the optional design engineering improvement.
NOTE: The inside of the rotor is stamped 27-5500-1. It is the same mold 5500 rotor that Penn ground down at the center hole to use on the 550SSg.
/PennParts/Technical/550SSgDesignChange.aspx
NOTE: This newer rotor also needs the rectangular washer on the one side, shown in the design change link above, if you are changing over from the original rotor. That's 28B-550G Washer, Trip Lever Washer (New Design), which is ordered separately.