Isabella of Castile to the Duke of Estrada, 20 August 1503

TASK: take a partial decipherment further: Isabella of Castile to the Duke of Estrada, 20 August 1503
This page: https://dbourdeau.github.io/cyphersolver/further/estrada1503.html

You are helping a public research project finish reading historical cipher letters. This one is read in part:
the key works, but some pieces are still open. Your job is to close one or more of the open gaps below.

THE READING
  Write-up:   https://dbourdeau.github.io/cyphersolver/estrada1503.html (read it first: how the text was read, the key, the reading)
  Date:       1503
  Outcome:    known key applied · partial
  Read now:   23.0% of the deciphered cipher tokens give sense (pieces below may be untouched)
  Files:      https://github.com/dbourdeau/cyphersolver/tree/main/targets/estrada1503 (NOTES.md, key, transcription, reading, scripts)

OPEN GAPS (from NOTES.md, "## Remaining gaps")
  1. [illegible: needs a new source] copy X recto ll. 12-13, 16-46 and the verso decoded, not read (mostly [..]; reading audit 22 Oct 2026; texts/PTR53-69_partial_reading.md 0.128 clean, 23 Oct) - blocker: illegible; the letter-sign runs cannot be told apart at PARES's 988 px (PARES returned HTTP 429 on 23 Oct, so not re-fetched); needs a higher-resolution copy from Simancas
  2. [illegible: needs a new source] Letter-sign runs, about half of the cipher tokens of each copy (154 [..] stretches in reading_X.md) - blocker: illegible; PARES serves only 988x1310 px, a sign is 8-10 px high and the alphabet has several homophones per letter; three copies collated; needs the AGS originals or a high-resolution reproduction from Simancas
  3. [illegible: needs a new source] X's day of the month and place ("en madri[d]?") and a few words of its endorsement - blocker: illegible; small cursive at 988 px

ALREADY TRIED (from NOTES.md, "## Escalation")
  [x] siblings: PTR 53/66-72 all opened; 53/69 holds three copies of the one letter (collated), 53/72 holds the clear copy of the 3 Oct letter that summarises it (used as a crib for sense and vocabulary).
  [x] clear-pages: no decipherment of the Galende-cipher letter on any image of 53/66-72; the clear copy on 53/72 ff. 346-349 is of the 3 Oct Estrada-cipher letter only.
  [x] known-keys: Galende's Cifra general fits (every code group consistent); Tomokiyo's "Estrada cipher" (cab, cev, gam, gep … raf, ru) does not occur.
  [x] print: CSP Spain i 369 (unread), Tomokiyo spanish.htm (no reading), Galende 1994 (key only); no edition found.
  [x] retry: (image enhancement, 22 Sept): every PARES route re-tested (zoom 10/20/100, print view, quality flags): max 997x1396, no IIIF or tiles. Strips at 4-6x with unsharp masking, read against Galende's plate (p. 167) of sign forms: adds isolated words (l. 12 *cud* una, l. 13 *tal resistencia … en ninguna dellas*) but the open runs stay ambiguous between two or three sign forms. A reading past ~50% is not possible from these images; the push to 95% is closed as impossible until Simancas supplies high-resolution photographs.
  [x] retry: X was read with Y (all 47 lines) and Z (first lines and closing) beside it; runs that read in one copy were carried over. A further sign-by-sign pass at this resolution would add little; better images are the step.

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/estrada1503/NOTES.md in full, and the
   key, transcription and reading files it names; lang/README.md (shared language models).
   Work in targets/estrada1503/. Add your findings to NOTES.md under a dated heading without rewriting what is there, and
   update the "## Remaining gaps" line of every gap you close. Keep WORKLOG.md there: one dated line per step
   (hypothesis, value tried, solver run, crib, control), failures included. Do not edit docs/ (the site is generated).
   If you cannot run a shell, fetch the files from the links above and keep your work in files you send with the issue.
1. Pick a gap. Start with the ones marked "doable now" (open-codes, not-attempted, left-open): the images and the key
   are online, so they need only work. A gap marked "needs a new source" (no-key-material, too-short, illegible,
   needs-physical-access) can be closed only by something new: a key
   or a deciphered sibling found elsewhere, a better image, a printed edition or calendar, a photograph from the archive.
   For those, search for that material first, and say where you looked even when you find nothing.
2. Start from the existing key and transcription in the folder. Do not rebuild the key from scratch, and do not
   re-read parts that are already read unless you find an error in them.
3. For a code group or sign value: list every place the group occurs, show how each reads with the value you propose,
   and say what still does not fit. For a passage: re-read it at the image and give the folio and line. For a missing
   letter or leaf: say where it is held and how it can be reached.
4. Measure again. Apply the key to the whole text, write the reading file, and count the share of cipher tokens that
   give sense (not an estimate). The reading is "complete" only when (1) at least 95% of its cipher tokens give sense;
   (2) no gap is left untried; (3) every letter in it is covered; (4) what stays open is scattered code groups or names,
   or pieces blocked from outside. Otherwise it stays "partial": say which gaps are still open and why.
5. Be honest. Mark every uncertain word with [?], leave unread groups as their numbers or signs, and never fill a gap
   with a plausible guess. A gap retried and still open, with what was ruled out, is worth reporting.

REPORT BACK
With a clone: open a pull request with your changes to targets/estrada1503/. Say in the description which gaps you worked on,
  the share read before and after, and how you want to be credited (name, initials or handle, and a link).
Without one: open an issue: https://github.com/dbourdeau/cyphersolver/issues/new?template=further-reading.yml&title=Further+reading%3A+estrada1503&page=estrada1503.html
  Give the write-up, the gap you worked on (its line in Remaining gaps, or the code group, folio and line), what you
  propose, the evidence (every occurrence, the images or sources), the share read before and after, your step log,
  and the tools and 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.