Where did the dressage letters come from?

“In the Old Imperial German Court the walls of the Royal Stable yard were initially marked with letters indicating where each Courtier was to be seated.”

dressage-arenaK Kaiser/King
F Fürst/Prince
P Pferdeknecht / Ostler
V Vassal
E Edeling / Ehrengast / Guest of Honor
B Bannerträger / Standard Bearer
S Schatzkanzler / Chancellor of Exchequer
R Ritter / Knight
M Meier / Steward
H Hofsmarschall / Lord Chancellor

But where are A, X and C??? The center line was added for the 1920 Olympics so the judges had a common map to make the judging consistent.