diff --git a/docs/micromate_protocol_reference.md b/docs/micromate_protocol_reference.md
index 326e6bc..ad2020b 100644
--- a/docs/micromate_protocol_reference.md
+++ b/docs/micromate_protocol_reference.md
@@ -964,12 +964,37 @@ block: it would return `PInstantel` on one unit and `VInstantel` on the other.
 Read the vendor at the fixed offset content[4]; the model is at content[26] on
 both lines, but anchor on `MM/` since only its tail varies.
 
-### Still not covered
+### ✅ Download from a BD unit (2026-09-30)
 
-- **A download from a BD unit.** UM20147 had no events stored, so the chunk
-  walk is verified on `11.0CB` only. Nothing suggests it differs — the `0x5A`
-  prefix and the offset arithmetic are not firmware-line dependent in anything
-  observed — but it is untested.
+UM20147, event `055d4a81`, 4,796 B, **type read as histogram from the `0x0C`
+byte and decoded as `.IDFH` on the first try.**  The chunk walk, the offset
+arithmetic, the 11-byte chunk prefix and the record-type byte all hold on the
+Thor firmware line unchanged.
+
+The type byte is now **7 events across both firmware lines** — 4 waveform,
+3 histogram — and has predicted correctly every time.  Still not a large
+sample, and the fallback stays.
+
+### 🔑 EVENT KEYS COLLIDE ACROSS UNITS — dedup must be (serial, key)
+
+UM20147's first event is `055d4a81`.  So is UM12947's.  **Different units,
+different sizes (4,796 B vs 4,076 B), different contents, identical key.**
+
+This confirms *keys are a sequential counter* (already recorded) and adds the
+consequence: **the counter starts from the same value on every unit, so a key is
+meaningless without its serial.**
+
+⚠ **Anything that stores, deduplicates or addresses Series IV events must key on
+`(serial, event_key)`.**  A store keyed on the event key alone will silently
+treat one unit's event as a duplicate of another's — and the failure is invisible,
+because the second event is simply never ingested.
+
+Series III has a related hazard the ACH server already handles: after an erase
+its counter resets, so keys are reused *within* a unit, which is why
+`ach_state.json` tracks `max_downloaded_key` per serial. Series IV inherits that
+and adds cross-unit collision on top.
+
+### Still not covered
 - **A unit that is monitoring**, and a unit with a nearly-full event buffer.
 - **The inbound call-home session** — still the one protocol unknown.