The classic "modern logic" game of WFF 'N PROOF uses a set of symbols to represent logical relations that I've seen used nowhere else: $C$ for then; $A$ for or; $K$ for and; $E$ for if and only if; and $N$ for not. These are used in prefix notation so that, for example $CNpq$ means $\neg p\Rightarrow q,$ and $EpAqNr$ means $p\Leftrightarrow \left({q\vee\neg r}\right).$
What is the origin of these symbols and have they been used widely or elsewhere? Are they still in use?