Among the Spanish ciphers in the reign of Philip II (for which see another article), Cp.30 is strange in that some of the substitution letters are the same as the plaintext letters. That is, "a" is enciphered as "a"; "b" as "b", etc. When I first saw it in Devos (1950), I thought it was a transcription error.
I found actual letters written in this cipher by Bernardino de Mendoza to Philip II and Martin de Ideaques, which were intercepted and deciphered (for the most part) on a separate sheet in BnF fr.3641 (see another article). They showed that the trivial substitution was actually used but the weakness was minimized because most syllables were enciphered with combinations of figures and vowel indicators.
The following shows a letter from Bernardino de Mendoza in Paris to Martin de Idiaques, dated 6 May 1590, in which yellow highlight shows where substitution is trivial.
Despite the trivial substitution, Cp.30 has its merit in providing indicators of consonants as well as vowels, which allows variations in enciphering (e.g., "tan" may be parsed as "t-an" or "ta-n" in enciphering).
Since the letter seems to discuss cipher, a transcription is given below, in which [ ] indicates words undeciphered in the manuscript but deciphered according to Cp.30 in Devos (1950) and "?" indicates where the deciphering does not match Cp.30. (Bearne must refer to Henry IV.)