diff --git a/docs/formats/core-data-regions.md b/docs/formats/core-data-regions.md
index 9c3fc06..ce97e3d 100644
--- a/docs/formats/core-data-regions.md
+++ b/docs/formats/core-data-regions.md
@@ -565,3 +565,34 @@ one. Neither applies here, and the tiling identity cannot be inflated at all.
 
 This is the strongest form of evidence produced in this project: a structural
 invariant confirmed by two methods that do not share a failure mode.
+
+## Sound bank SBlk: a 12-byte marker that does not explain the 40-byte period
+
+**Range measured:** level 0 `SBlk`, 24..19508 (19,484 bytes).
+
+**Method:** positions of the `0x7f`/`0x47f` marker words, and their spacing.
+
+**Result:** those markers sit at **12-byte spacing on 144 of roughly 180
+occurrences**, with 24-byte and larger gaps accounting for the rest. So there
+is a 12-byte structure in the data.
+
+**But that contradicts the autocorrelation.** If records were 12 bytes, stride
+12 should be the peak. It is not:
+
+| stride | excess over shuffled |
+|---|---|
+| 12 | +0.237 |
+| 20 | +0.175 |
+| 24 | +0.237 |
+| **40** | **+0.454** |
+| 48 | +0.239 |
+
+40 is not a multiple of 12, so the two measurements disagree rather than one
+explaining the other. Recorded as an open discrepancy rather than resolved in
+favour of either: the marker spacing says 12, the period says 40, and 40 being
+twice as strong is the more surprising of the two.
+
+This is the first place in this project where two sound measurements of the
+same data point at incompatible answers. Worth resolving before any record
+layout is attempted, because a layout built on the wrong one will parse
+plausibly and be wrong.