diff --git a/docs/re/structures/regn-map-grid.md b/docs/re/structures/regn-map-grid.md
index ca41a5b0..3dcd96c3 100644
--- a/docs/re/structures/regn-map-grid.md
+++ b/docs/re/structures/regn-map-grid.md
@@ -193,11 +193,60 @@ and consistent with `MCOL` sitting beside it.
 
 ### ❔ Still open
 
-* **Section 1** (96 bytes/record, 60 631 records disc-wide) is untouched. Its
-  content is float-dominated with both large coordinates and mid-range values.
-  96 bytes is 24 floats — a plausible shape for a 4×4 matrix plus extras, or for
-  two 48-byte plane records, but nothing is measured.
+* **Section 1** (96 bytes/record, **63 410** records disc-wide) — a per-slot
+  census is below, but what the record *means* is still ❔.
 * **What the planes are for.** "Collision or region boundary" is a reading of the
   shape; nothing here shows what queries them.
 * The **zeros at `[0..3]`** of every plane record, and the constant `96`-byte
   tail after section 2, are unexplained.
+
+
+## 🟡 Section 1, slot by slot — measured, but not read
+
+**2026-08-26.** 96 bytes is 24 slots. Censusing every one of the **63 410**
+records across all 11 objects gives a clear regional structure, even though the
+record's purpose is not settled:
+
+| slots | what the values are | reading |
+|---|---|---|
+| 0–3 | slots 2 and 3 are zero in ~100 %; 0 and 1 mostly small | ❔ |
+| **4–6** | 96 % have \|v\| > 1 000, range **±250 000** — the header bbox range | 🟡 a **position** |
+| **7** | 99.9 % > 1 000, always positive, 519 … 107 600 | 🟡 a **radius or extent** |
+| 8–11 | as `f32` these are **denormals** (1.4 × 10⁻⁴⁵ upward) — i.e. they are **integers**, not floats | ✅ integer fields |
+| 12–23 | six **pairs**: the even slot 200 … 67 000 and never zero, the odd 0 … 212 000 and zero in 7–13 % | ❔ |
+
+The denormal signature in slots 8–11 is worth stating plainly: a float field
+never holds `1.4e-45`, so those four words are integers that a float reader
+would silently turn into near-zero garbage.
+
+A position plus a positive scalar plus integer links is the shape of a
+**bounding-volume hierarchy node**, which would fit a file that also carries a
+point list and a plane list. That is a reading of the shape and nothing more.
+
+### ❌ The index test has no discriminating power
+
+I tried to confirm the integer slots are indices by splitting each into two
+`u16`s and checking them against each section's record count. The result is
+useless, and the reason is worth recording:
+
+| slot | halves valid for section 0 | section 1 | section 2 |
+|---|---|---|---|
+| 8 | 100.00 % | 100.00 % | 100.00 % |
+| 9 | 100.00 % | 99.99 % | 100.00 % |
+| 10 | 8.78 % | 44.19 % | 100.00 % |
+| 11 | 8.76 % | 44.16 % | 100.00 % |
+
+**A test that accepts every hypothesis rejects none.** Section 2 has tens of
+thousands of records, so "is this `u16` below the plane count" is satisfied by
+almost any small number — it measures the size of the section, not the meaning
+of the field. Only slots 10 and 11 discriminate at all, and they merely rule
+*out* section 0.
+
+`slot 11`'s halves are consecutive (`n`, `n+1`) in 54 % of records and slot 10's
+in 12 % — suggestive of paired links, but 54 % is not a rule and I am not
+promoting it.
+
+❔ So section 1 stays open. What would settle it is a test with power: pick a
+record, follow a candidate index, and check that the thing it lands on is
+spatially consistent with that record's own position and radius. That needs the
+tree walked, not the fields counted.
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