An undeciphered letter from Le Tellier to Marquis de Castelnau, dated 12 May 1657, is presented in "Ciphers Early in the Reign of Louis XIV". From other letters in Le Tellier's correspondence at the time, it is considered to be in a syllabic cipher. The ciphertext spanning a little over half a page seemed too short for a conventional solver for general syllable assignment.
To my pleasant surprise, it was solved by Robert Pitt. I guess the regularity of the assignment was given as a constraint or training data. The solution is described in a new short article, "Solution of Le Tellier-Castelnau Cipher (1657)".
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