BnF fr. 15562 (Gallica) contains two letters mostly in cipher (ff.65-66 and ff.69-70). The second letter is deciphered in ff.67-68 but apparently the first letter remains undeciphered. I reconstructed the cipher and could (more or less) reveal the plaintext of the first letter, which turned out to be information from Antwerp, dated 16 January 1580.
It was probably written by "Monsieur de Revers (Reuers)", the writer of the second letter, who seems to be the person who had been sent by the French King to negotiate with the Prince of Orange in 1576 and 1578 about French support (William Herle to Lord Burleigh, 16 March 1576 (pdf, p.272, 407), Catalogue of the Harleian Manuscripts in the British Museum, vol.1, p.145, no.73; The name is also mentioned in Cecil Papers, October 1575 as well as in a letter (1575) from Herle to Burleigh in SP 70/137/[52]f.228r-229v (pdf).)
Antwerp was then the capital of the Dutch provinces because the States General moved there when Brussels was threatened by the Spanish army in 1578 (Parker p.186; Correspondance p.xcviii). During the session of the winter of 1579-1580, the Prince of Orange, leader of the Dutch Revolt, was frustrated with the slow proceedings in the States General. He gave a remonstrance to the deputies (9 January 1580), and soon left for the North provinces (Kossmann; Correspondance vol.4; Wedgewood p.208). It appears to have been occasioned by loss of Saint-Amand and Mortagne, mentioned in a despatch of the States General to Villers, Governor of Bouchain, of 11 January 1580 (Actes p.321).
Facing the threat of the Spanish army under the Duke of Parma, the Dutch needed a foreign alliance. While the States General had selected Archduke Matthias (brother of Rudolf II, Holy Roman Emperor) as Governor-General of the Netherlands under the nominal rule of the Spanish crown, now that reconciliation with the king seemed hopeless, the Prince of Orange and his confidant Saint-Aldegonde were seeking alliance with the Duke of Anjou (called Monsieur), brother of Henry III of France. (Roch de Sorbiers, sieur des Pruneaux, represented the Duke before the States General since 1578 (Duquesne p.29; Actes p.8).) There was talk of the Duke of Anjou's marriage to Queen Elizabeth of England, which, if realized, would have increased the importance of the Duke. However, there were people who suspected the Prince of Orange was seeking his own promotion through alliance with France. It was only in August 1580 that the States General agreed on seeking protection of the Duke of Anjou by offering sovereignty of the Netherlands. (Parker p.197, Wedgewood p.215)
Before the States General made a decision, Cambrai accepted protection of the Duke of Anjou in October 1579. (Actes no.2067, Duquesne p.86)
The reconstructed cipher is as follows.
Reconstructing the cipher was more trouble than it should have been because I could not determine whether the decipherment on ff.67-68 corresponds to the first letter or the second letter. I was not aware of the correspondence between the endorsement of the decipherment "dechiffrem[ent] dune l[ett]re de Mons de Reuers de 18 de Janvier 1580" on f.68v and that of the second letter "Mr de Reuers du - Januier 1580" on f.71v. (I could not even read the handwriting.)
So, at first, I had to rely on a few interlined letters in the first letter in reconstructing the cipher. However, the letters were hardly legible and I could not distinguish between e/u/n, between r/s, between a/u/n, etc. and my first identifications later turned out to be wrong. (I first thought the "2"-like symbol was "i" but it turned out to be "e". I first thought the "m"-like symbol was "n" but it was "u/v".) The first interlined word I could read was "chauuin" on lines 1-2 of f.66. It was consistent with what looked like "avec." Then, another interlined plaintext "croist" could be read. This revealed sufficient symbols with which to identify further symbols.
For example, when a pattern "e*eche" was found, the unknown symbol could be inferred to be "p". (What I thought to be "depeche" turned out to be "empecher", though.) This was consistent with another instance "*lus" and was confirmed when a pattern "de*utes" was found. Less frequent letters could be identified by occurrences of patterns such as "*orces" (*=f), "*iminuer" (*=d), "ne*otiation" (*=g), "e*tremant" (*=x), etc.
Only after much of the plaintext of the two letters could be deciphered, I noticed that the decipherment of f.67-68 corresponded to the second letter. Comparison allowed identification of the meaning of code symbols representing "Pays Bas", "Flandre", "France", etc., while some (representing "prince" (de Conde), "roi" (de Navarre)) had been identified before that.
Even with the reconstructed cipher, the interlined text corresponds to where the deciphered text is awkward. From my experience, when one gets accustomed to this kind of simple cipher, one can translate the ciphertext into plaintext without much trouble. Perhaps, the interlined text shows where the deciphering clerk had to stop to read letter by letter to find out the plaintext word because of some unfamiliar words or spelling.
At the time, the spelling convention was not the same as what is common today. In the first place, there was no standard spelling. (See another article.)
Even allowing for this, the spelling of this writer seems out of standard.
In particular, "ai" in modern orthography (or "oi" at the time) is often written as "e" (e.g., "pourrait" on f.69, line 3 of the ciphertext vs. f.67v, line 6 of the decipherment; "fallait" on f.70v, 7th line from the bottom vs. f68v, 4th line from the bottom).
"Huguenot" is often spelled without "h" (e.g., "noblece uguenote" on the 19th line from the bottom of f.69 vs. the 13th line from the bottom of f.67).
These posed trouble in the initial attempts in my reconstruction work.
The following is the first letter (more or less) deciphered.
Actes des Etats Generaux des Pays Bas vol.2 (Google)
E.H. Kossmann, A.F. Mellink (1974), "Texts concerning the Revolt of the Netherlands" (DBNL)
Correspondance de Guillaume le Taciturne, prince d'Orange vol.4 (1854) (Google)
Frédéric Duquesne (1998), Pays-Bas de 1580 a 1584 (Google)
Geoffrey Parker (1977, 1985), The Dutch Revolt, Revised Edition, Peregrine Books
Jonathan I. Israel (1995, 1998), The Dutch Republic, Clarendon Press
C.V. Wedgewood (1944, 1968), William the Silent, The Norton Library