diff --git a/TODO.md b/TODO.md
index de60815..b823b67 100644
--- a/TODO.md
+++ b/TODO.md
@@ -785,11 +785,11 @@ not obvious from any one of them.
       | Hybrid Rifleman RFL-3N (Sneede) | def -3.6 | one-off, two variants |
       | Hercules HRC-LS-9004 | def +1.3 | Ultra AC/10 and an AMS |
 
-      The strongest lead is the ArtilleryMech, because the figure is exact: our
-      offensive rating is 118 where MegaMek's is 161, and the Thumper it
-      carries is worth 43. Something counts that weapon twice, or counts it
-      once somewhere helm does not look - artillery is the obvious suspect,
-      since nothing else in the library is scored through the same path.
+      The ArtilleryMech is settled, and it is not our arithmetic. Its file
+      declares two Thumpers and allocates fifteen critical slots between them,
+      which is one Thumper's worth: the design cannot be built as written.
+      MegaMek scores two and helm reads the criticals and finds one, which is
+      the 43 points exactly. Written up in `crates/helm-bv/UPSTREAM.md`.
 - [x] **What a recompute costs.** `helm bv-report --bench`, because "can the
       browser do this as somebody drags a slider" needed an answer before the
       wasm work is designed around it. The three things a player does are not
diff --git a/crates/helm-bv/UPSTREAM.md b/crates/helm-bv/UPSTREAM.md
index ee74bda..e228990 100644
--- a/crates/helm-bv/UPSTREAM.md
+++ b/crates/helm-bv/UPSTREAM.md
@@ -176,3 +176,19 @@ Found by running helm's upload check over every `.mul` the install ships: 99
 machines, and these are the only four that name a design the library does not
 have. The fix is `model=""`, which is what the other entities in the same file
 do for a base variant.
+
+## The Uni ATAE-70 ArtilleryMech is allocated one Thumper and carries two
+
+`meks/.../Uni ATAE-70 ArtilleryMech.mtf` declares `Weapons:2`, naming a Thumper
+in the centre torso and another in the right torso. Its critical slots hold
+fifteen `ISThumper` lines - seven in the left torso, seven in the right, one in
+the centre - and a Thumper takes fifteen slots each. So the file allocates one
+Thumper's worth of slots to two Thumpers, and neither of the placements in the
+weapons block matches where the slots actually are.
+
+The design cannot be built as written: thirty slots are needed and fifteen are
+present. MegaMek scores it as carrying two, which is where helm's 43-point
+disagreement comes from - helm reads the criticals and finds one.
+
+Which half of the file MegaMek believes has not been established here. What is
+plain from the file alone is that the two halves disagree with each other.