Daniel Bourdeau solved "French Cipher Despatch received by General Catinat (1691)" in my list of "Unsolved Historical Ciphers".
He has been solving so many items in the past few days (he told me he solved 26, with avg 1-2 hours per problem) that I cannot catch up in updating my list. So I put a notice in my list that people about to start working should first check his website. As far as I have seen, his solution and analysis are of very high quality.
For this particular case, automatic codebreaking by AI failed. He found another ciphertext in the same cipher with a paraphrased translation, in which matching ciphertext-plaintext portions could be found from recurring symbols.
See "Solution of Louvois's Petit Chiffre (1690-1691)" and his github quoted therein.
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