The report is attached below, from which the conclusion reached is that:

Under explicit same-column filler models, the observed final-18 pull-down structure is substantially more likely than under the tested ordinary continuation models. Depending on the continuation baseline, the estimated likelihood ratio ranges from roughly tens-to-one to thousands-to-one. This supports deliberate or semi-deliberate filler construction over ordinary continuation text, while remaining dependent on the assumptions of the generative models and without proving authorial intent.

Whether or not we can rest satisfied this result, even if only mildly so, is then the next question.

But, hold on. Have we not just taken our control analysis too far in the reverse direction? Is it really as damning to our original control analysis as stated here?

The problem now is that we have actually introduced an unwarranted 'circularity' into the equation.

If the final 18 scores highly because it contains qEHM, and qEHM is copied from another line, then a symmetric control that allows copying in either direction will naturally tend to give the source line credit for the same relationship. In other words, the symmetric control is partly counting the same pairwise resemblance twice.

So now we have to eliminate this latter issue, and we can do so by formulating a 'pairwise analysis', with the intention of excluding from the results any that are merely mirror-copies of the proposed last 18 clustering anomalies. Running this revised analysis we discover, as we have suspected, that:

The previous “higher” symmetric control was indeed the mirror-image of the final-18 relationship.

Specifically:

Final 18 contains:      qEHM
Source in earlier grid: row 8, col 9

Symmetric control then allowed:
row 8 to copy qEHM back from row 24

So that control was not an independent rival anomaly.

When I excluded the final rows from acting as sources for non-final control windows, the results became:

Final 18 score:       26
Next highest control: 12
Next:                 10
Next:                  9
Next:                  8

So the final 18 returns to being the strongest boundary-crossing window once the circular mirror-copy is removed.

Does the final line of the Z408 exhibit a degree of vertical copying from previous rows that would be unlikely to occur by chance?

we find, ultimately, that we are required to pull somewhat back from seeking to answer this question positively, having been reduced to determining merely that:

“The final 18 contain a pattern consistent with having been generated by copying previous cipher material.”

or, as it may be alternatively stated:

“Given that the final 18 are already known not to behave like normal ciphertext, their internal copying structure provides a plausible explanation for how they were generated.”

Whether or not we can rest satisfied this result, even if only mildly so, is then the next question.

The Monte Carlo test ... asks how often a shuffled Z408-like grid gives the terminal 18 position a pull-down score at least as high as the real one when copying only from previous material.

On a zero-drift run (pull-down from the exact same column), we again get a random mean score of 6.510 from 30,000 runs with a maximum random score of 20, the plus-one p-value unchanged at 0.000033.

These figures support the same basic conclusion as before: under a frequency-preserving shuffle model, the final 18 remain unusual at the terminal position.

However, not everything is thrown away with this important amendment, as the 'line comparison' analysis is not the primary arbiter of the actual last 18 which, by their very nature - and fully in line with the actual 'pull-down' hypothesis - must be situated at the end of the ciphertext.

ChatGPT then offers the following "revised interpretation":

"The amended analysis changes the internal-control interpretation. The final 18 are not uniquely superior once other boundary-crossing windows are given equal access to above and below rows. One earlier non-final control scores 28, slightly above the final 18 score of 26.

"However, this does not erase the main historical finding. The final 18 still score highly, and the Monte Carlo final-position test still finds that a terminal pull-down score of this size is rare under simple frequency-preserving randomisation.

"The most balanced conclusion is therefore: the final 18 show a statistically unusual terminal pull-down structure, but the symmetric controls reveal that this structure is partly driven by a repeated qEHM cluster also present elsewhere in the cipher. The evidence supports deliberate filler construction more than ordinary continuation text, but it should no longer be described as uniquely unmatched by internal controls."

The final row was therefore not merely highest, but separated substantially from the rest of the cipher.

must, of itself, be considered to be potentially (even likely) misleading, in having resulted from demanding that:

Every other row was tested as if it were the unexplained ending.

At the very least, we need to run a line comparison test that utilises the remaining 23 lines in all instances, last line or not.

We cannot absolutely discount hidden text, but the pull-down evidence shifts probability away from “unsolved continuation” and toward “intentional filler constructed from existing cipher material.”

Further testing is indicated, which we are encouraged to pursue.

A possible further test

There is actually another analysis that follows naturally:

Ask:

Are the final 18 statistically consistent with the symbol-selection behaviour of the first 390?

For example:

• symbol frequency,
• rare-symbol usage,
• homophone distribution,
• repeated bigrams/trigrams,
• entropy.

If the final 18 behave unlike normal enciphered Z408 text but like copied fragments, that would strengthen the padding interpretation.

A particularly interesting test would be:

Take every 18-symbol segment in the solved portion and ask:

Does any genuine ciphertext segment show this level of vertical self-copying?

If genuine encrypted text almost never does, the inference becomes stronger.

Under this model, the Z408 final row appears to be a genuine statistical outlier. A fair summary would be:

The final symbols of Z408 contain a vertical copying structure that is unlikely to arise from random placement of the same cipher symbols. This supports the plausibility of a deliberate pull-down construction, although it does not by itself establish authorial intent.

This does not prove that Zodiac intentionally created the final symbols by copying.. Alternative explanations remain possible:

• the scoring system may not perfectly model chance resemblance;
• another null model might produce different results;
• the final row was selected because it was already known to be unusual.

The important safeguard is that random trials were given the same opportunity to find their own “best” pull-down patterns.