diff --git a/docs/formats/core-data-regions.md b/docs/formats/core-data-regions.md
index 4c8557f..5cf12b9 100644
--- a/docs/formats/core-data-regions.md
+++ b/docs/formats/core-data-regions.md
@@ -617,3 +617,26 @@ The earlier "they disagree" framing was wrong in one respect: the marker
 spacing never claimed to be the record size. Read as a claim about markers
 inside records rather than about the record itself, the two measurements were
 never in conflict.
+
+## SBlk record layout at 40 bytes
+
+**Range measured:** level 0 `SBlk`, 24..19508, aligned at +0 — 487 whole
+records.
+
+**Method:** for each of 40 candidate alignments, record the number of byte
+positions that are near-constant across records, and keep the best.
+
+**Result:** alignment +0 is best, with 10 near-constant positions. Four are
+exactly constant and all zero: positions **7, 11, 31 and 35**.
+
+Distinct values per byte position across the 487 records runs from 1 to 155,
+with position 29 the most variable by a wide margin (155 against 21-48
+elsewhere) and positions 2, 10, 27, 34 and 39 low (2 to 8).
+
+The first record carries the `SBlk` magic and the `DAW` tag rather than
+descriptor data, so it is a header rather than an entry — which is worth
+accounting for before reading any field at a fixed offset.
+
+**Not decoded.** The layout is established and the constant positions are
+known; the fields are not named, and which of the 40 bytes are an index, a
+length and an offset is still open.