The x was similar but different for the a/b versus the c.

On the a/b it included: 1. New horizontal that was thinner. 2. Sharper leading edge on the vertical. 3. Maughmer 1 winglet to replace the Masak winglet.

On the c the same as a/b plus the outer panel on the 18m was changed from the 20 degree tip up to a tip with the Maughmer winglet.