Wallis's 'challenge' letters nos. 50, 51 and 53

TASK: help read Unsolved Catalogue no. 470 of the cyphersolver project
This page: https://dbourdeau.github.io/cyphersolver/solve/470.html

You are helping a public research project read historical cipher letters that nobody has read yet.

START HERE: the project is already working on this cipher. Read these notes before anything else and
build on them; they record what was transcribed, tried and ruled out, and the next step:
  https://github.com/dbourdeau/cyphersolver/blob/main/targets/wallis1653/NOTES.md  (status: in progress)

THE CIPHER
  Title:          Wallis's 'challenge' letters nos. 50, 51 and 53
  Date:           1651-1658 (four letters; Brasset 4 Apr 1653)
  Correspondents: Henri Brasset → Antoine de Bordeaux-Neufville; George Villiers, 2nd Duke of Buckingham → unknown; Peter Townesend → unknown; unknown → unknown
  Place:          The Hague; Flanders; Scotland; unknown
  Language:       French (EMLO: 'not deciphered and is in French') (expected)
  Held at:        Bodleian Library, MS. e Mus. 203 pp. 218-226; copies in MS. Eng. misc. e. 475 pp. 234-247 and 'MS. 19336' pp. 203-219 (EMLO's manifestation shelfmarks); Letters 50-53 of Wallis's 'Collection of letters intercepted in cipher' (1653): no. 50 Brasset → Bordeaux 4 Apr 1653 (numeric + symbol substitution), no. 51 unknown, Scotland, 16 Jan 1651, no. 52 Buckingham, undated, no. 53 Peter Townesend, Flanders, 5 May 1658 (numeric substitution). EMLO works cb69093b, 3329a9ec, 86e81b88, eef01c0b.
  Images/records:
    - EMLO letter 50 (Brasset 1653): https://emlo.bodleian.ox.ac.uk/profile/work/cb69093b-a032-422e-b21a-ab88c97353e0
    - EMLO letter 51 (Scotland 1651): https://emlo.bodleian.ox.ac.uk/profile/work/3329a9ec-d1f2-46f7-8d62-6d5c52e29780
    - EMLO letter 52 (Buckingham): https://emlo.bodleian.ox.ac.uk/profile/work/86e81b88-5ddf-4853-a4c1-6b19fd2b3785
    - EMLO letter 53 (Townesend 1658): https://emlo.bodleian.ox.ac.uk/profile/work/eef01c0b-7cf0-43b8-b214-bac802f5a6aa
    - Dominic Winter lot 375 (2020): https://www.dominicwinter.co.uk/Auction/Lot/375-wallis-john-a-collection-of-letters-intercepted-in-cipher-during-the-late-warres-in-england-1653/?lot=361781
    - BL Add MS 32499 f. 1 (d'Aumont cipher, Dec 1651): https://searcharchives.bl.uk/catalog/032-002026538

WHAT IS KNOWN
  Status: EMLO notes for each: 'The letter is not deciphered; however, the cipher appears to be based on the substitution of numeric values.' The auction copy of the same collection (Dominic Winter, 7 Oct 2020, lot 375) says the last four letters are 'left unsolved as exercises for aspirant cryptographers'. The other 49 letters carry Wallis's decipherment and key, so related Royalist/French keys sit in the same volume. Not on DECODE under these shelfmarks (known.py: no hit for e Musaeo 203 / Eng. misc. e. 475 / Brasset 1653; DECODE has a different Brasset record R9429, Oct 1648). Narrowed to the unread part and restored 2 Nov 2026.
  Outcome so far: In progress here, in targets/wallis1653/.
  Prior-art check: no. 52 (Buckingham) already deciphered in print: John Davys, An Essay on the Art of Decyphering (London 1737) pp. 24-29, interlinear decipherment + key, 456/480 groups (transcribed and checked 27 Oct 2026, targets/wallis1653). Davys counts three undeciphered letters in the Bodleian copy (one French, two English = nos. 50-52); no. 53 (Townesend 1658) is in the private 2020 copy. Nos. 50, 51, 53 open as far as checked (web, cryptiana, Thurloe SP i); not digitised, blocked on Bodleian access. cryptiana's 'French cipher unsolved by Wallis' is a different 1690 item, solved by Biermann 2020.
  Check first: Order or view images of MS. e Mus. 203 pp. 218-226 (Digital Bodleian has no images listed in EMLO); check the 2020 auction copy description and whether Wallis's own keys for the French letters (Brasset/Bordeaux) in the same volume fit no. 50. Also test d'Aumont's Dec 1651 cipher, 'pareil à ceux' sent to Mme d'Estrades and Brasset (BL Add MS 32499 f. 1, Wallis's letter-book; pointer from Michael Brackx, issue #117), together with Bordeaux's R7537.
  Class B: part of a key or a known cipher family of the same office is in print or on DECODE.
  Project notes (read these before anything else; they record what was tried and ruled out): https://github.com/dbourdeau/cyphersolver/blob/main/targets/wallis1653/NOTES.md

STEPS
0. Set up. If you can run a shell, fork and clone the repository and work inside it:
     git clone https://github.com/dbourdeau/cyphersolver.git
   Then read, in this order: CONTRIBUTING.md (how a contribution is sent and credited); README.md, sections
   "Repository layout" and "Conventions" (how readings are measured and named); targets/wallis1653/NOTES.md (everything
   already tried on this cipher, with the files it used); and lang/README.md (shared language models).
   Put all your work in targets/wallis1653/: transcription, key, decipherment, scripts. Add your findings to its NOTES.md
   under a dated heading, without rewriting what is there. Keep WORKLOG.md there too:
   one dated line per step (transcription, hypothesis, solver run, crib, key found, control), failures included.
   Do not edit docs/ (the site is generated and rebuilt on merge) and do not commit large images: link them.
   If you cannot run a shell, do the same work in files you keep and send them with the issue below.
1. Make sure it is still unread. Search the web and the literature for the shelfmark, the correspondents and the date:
   printed editions and calendars of the correspondence, Satoshi Tomokiyo's cryptiana pages
   (https://cryptiana.web.fc2.com/), the DECODE record and the files attached to it, and the project notes above.
   If a decipherment already exists, stop and report where it is. That is a useful result.
2. Get the images. For Gallica items, a page image is
   https://gallica.bnf.fr/iiif/ark:/12148/<ark>/f<page>/full/full/0/native.jpg (page numbers from the manifest).
   DECODE records (de-crypt.org) need a free account: if you cannot log in, ask your human to download the images for you.
3. Look for key material before attacking the ciphertext, in this order: a decipherment written between the lines or
   in the margin; a clear copy or a deciphered sibling letter in the same volume or DECODE series; a key sheet from the
   same office and years (DECODE key records, Tomokiyo's articles, printed editions); clear passages in the letter
   itself that can serve as a crib.
4. Transcribe exactly what is on the page: one token per sign or number group, line by line, with folio and line
   numbers, unclear signs marked with ?. Keep the transcription as a file. It is worth sending even if nothing else works.
5. If no key exists, identify the system (count the distinct symbols, tokens and repeats: simple or homophonic
   substitution, nomenclator with code words, syllabary, two-digit figures) and recover the key from the ciphertext
   with a solver scored on a language model of the period (French (EMLO: 'not deciphered and is in French')). In the clone, use the shared models in lang/
   (from lang import lm; lm.load('<model>'), lm.best_language(text)) rather than building your own; add a corpus there
   if none fits. Before trusting a result, run the same solver on a control text you encipher yourself in the same
   system and at the same length. If it cannot solve the control, its output on the real letter means nothing.
6. Measure the result against the project's bar (README.md, Conventions). A text is "complete" only when (1) at least
   95% of its cipher tokens give sense, counted from a reading file, not estimated; (2) no gap is left untried;
   (3) every letter in the item is covered; (4) what stays open is scattered code groups or names, or pieces blocked
   from outside (no key material, too short, illegible, needs the archive). Otherwise it is "partial": say which
   pieces are open and why. Mark every uncertain word with [?] and leave unread groups as their numbers or signs.
   Never fill gaps with plausible guesses. A failed attempt that records what was ruled out is still a contribution.
7. Name how the text was obtained with one of the project's methods (do not write just "solved" or "read"):
     - ciphertext-only break: The key was rebuilt from the ciphertext alone: frequencies, solvers, pattern words, the order of the key. No plaintext and no existing key was used. If someone had solved the same text earlier and it was found only afterwards, the write-up says so: an independent re-solution.
     - key from external plaintext: The key was rebuilt by matching the ciphertext against a plaintext found elsewhere: a printed edition, a calendar of state papers, a copy of the same letter in another archive.
     - key from adjacent plaintext: The key was rebuilt from plaintext next to the ciphertext: a partial decipherment written between the lines or in the margin, clear passages in the same letter, a deciphered copy in the same file.
     - external key matched: An existing key that had not been connected with this document (a key for another correspondent or series, a key sheet filed elsewhere) was found to fit, and applied.
     - known key applied: The key was already identified for this document or its correspondence (on its archive or DECODE record, published for it, filed with it) and was applied. The work is transcription and decipherment, which is not mechanical when the key is polyphonic or variable-length.
     - existing decipherment: A complete decipherment already existed (between the lines, on a separate copy, or in print). The work was transcribing the cipher and checking the decipherment against it.
     - not solved: Attempted; neither the key nor the text was obtained. What was learned is recorded. A target whose key was obtained but whose text does not read keeps the method that obtained the key, with extent "none (text not read)".
     - not applicable: Not a cipher (clear text, shorthand, a key sheet, instructions), or the item could not be reached.

REPORT BACK
With a clone: open a pull request with your targets/wallis1653/ folder. Say in the description how you want to be
  credited (name, initials or handle, and a link) and include the same points as the issue below.
Without one: open an issue: https://github.com/dbourdeau/cyphersolver/issues/new?template=solution.yml&title=Solution%3A+catalogue+470&target=catalogue+470
(other kinds of contribution: https://github.com/dbourdeau/cyphersolver/issues/new/choose). Include:
  - Target: catalogue 470
  - the documents and the images you worked from;
  - the key: sign-to-value table, code groups with values, nulls;
  - the decipherment line by line against the ciphertext, with uncertain passages marked;
  - the method (step 7), the share of tokens that give sense, complete or partial (step 6), and whether the
    reading was already published anywhere;
  - your step log (WORKLOG.md), and the tools and the AI model you used.
If you cannot open issues or pull requests, write this report for your human to send. Credit goes to the person who sends it.