OK here goes....Forget everything you think you know about the "current" SCTA classes.. Back in 1958 for sure (I'm not sure for how long or when it started) everything was alphbetical.
It just happened that starting with "A" were OHV V-8 Coupe and Sedan. "A" was 183" to 270", "B" was 271" to 295", "C" was 296" to 330", and "D" was 331" to 399". E,F,G,H,I,J and so on continued on for different styles of vehicles and different engine sizes. Most everything was restricted 400" unblown and 300" blown.
Since I'm not sure when it started or how long it kept going; at some point in Coupe and Sedan you were allowed to have a larger than 399" engine. Since "E" was already used a "X" was added after the "D" class for DX meaning a Coupe and Sedan over 400".
I looked at a sheet of all the classes in 1958 and they went to "W"....................There you go.......JD