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?
It's polish notation. For example, $K$ stands for koniunkcja, from the same root as "conjunction".