There are plenty of tales and myths around the CSX chassis numbers and what the letters stood for.
Whichever story you happen to believe regarding the chassis numbers, there is a chassis prefix to throw a spanner in the works. I suspect that some of the prefixes were made up as they went along, with very little thought to the logic.
Following previous AC convention, the Mk.III chassis cars (coil-sprung) should probably have been a "D" prefix i.e. DSX#### - D series, S Shelby, X Export. Instead, they broke convention and went up from 2### to 3###. Maybe Shelby had something to say on it because he had been telling everyone the CS stood for Carroll Shelby.
Paul