Encryption

I'm sure it showed up in Hardy Boys. It's not that unbelievable.

I was a nerdy kid, and I played numerical crosswords. I had read Kjartan Poskitt’s books including Codes: How to Make Them and Break Them (in Czech, of course). I only remembered the first few vowels and one consonant from his list of top characters but that was enough because

• I had enough time for guesswork (took me about 90 minutes, I could do it in about half the time now with more experience)
• there were recurrent words relating to the theming of the camp
• Czech letter frequencies are more lopsided than English (we barely use F, G, Q, W, X)
• one page is good enough to get good statistics, and the two or three top letters did match expectations
• I could technically somewhat accurately recreate Kjartan Poskitt’s (actually the translator’s) frequncy table using a Harry Potter book I had (turned out not to be neccessary, it would have been time-consuming)

Some cipher creators actually preserve diacritics. Seeing a symbol like έ, ≗ or ⊔̌ would be a giant giveaway because we only have the following diacritics: ČĚŘŠŽ (+ rare ĎŇŤ), ÁÉÍÝ (+ rare ÓÚ), Ů. The organizers had converted the plaintext to ASCII before encrypting it but this is still weaker than treating letters with diacritics as completely separate.

I don't think my story is too unbelievable. Some kids just learn things faster/slower than others. I learned to write in all caps before I could wipe my butt (I'll leave it to interpretation if I was an early writer or late-ass wiper).