diff --git a/restoration/source410/BT/MECH-LAYOUT.md b/restoration/source410/BT/MECH-LAYOUT.md
index c453663c..45abf970 100644
--- a/restoration/source410/BT/MECH-LAYOUT.md
+++ b/restoration/source410/BT/MECH-LAYOUT.md
@@ -64,3 +64,57 @@ binary alloc size is WRONG and must be fixed before use.
 2. Untangle the three offset CONFLICTs above (decomp reads).
 3. Write the first staged MECH.HPP (interface + mapped runs + reserved[] filler) and
    compile-verify sizeof(Mech)==0x854 with the probe.
+
+---
+
+## ADDENDUM 2026-07-19 (later) — spawn factory reached the Mech ctor; proxy layer resolved
+
+The boot ladder now runs the full spawn factory (CreatePlayerVehicle -> Mech::MakeMessage ->
+MakeAndLinkViewpointEntity -> registry -> Mech::Make -> `new Mech` -> JointedMover base
+streams the model) and halts at the Mech ctor. So the ctor is the live frontier and this
+worksheet is the active blocker.
+
+### Proxy layer resolved (BT411 mechrecon.hpp -> real 1995 types)
+
+Settles the open item in the "Real 1995 types" bullet above:
+
+| BT411 alias | real type | in 4.10 archive? |
+|---|---|---|
+| ReconAlarm (AlarmIndicator) | AlarmIndicator (multi-level, N levels sets size) | ❌ MISSING |
+| ReconSeq | SequenceController (animation sequence) | ❌ MISSING |
+| ReconFiltered / FilteredScalar | AverageOf<Scalar> (15-sample running avg) | ❌ MISSING |
+| ReconFiltered (mechNameFilter) | NameFilter | ❌ MISSING |
+| ReconChain<Subsystem*> | ChainOf<Subsystem*> | ✅ CHAIN.HPP |
+| ReconMatrix / Matrix34 | AffineMatrix / Matrix | ✅ |
+| StateIndicator | StateIndicator | ✅ STATE.HPP |
+| Reticle | Reticle | ✅ RETICLE.HPP |
+| BTVal (turretBase, mechName, torsoAim*, aimRate, headPitch, torsoTwist) | animated value/quaternion — **UNIDENTIFIED**; BT411 BTVal is a no-op placeholder. Most BTVal members are < 0x328 = BASE-class pose fields (re-attribute per §"Consequences"), so their type is a base concern, not Mech's. |
+
+### CRITICAL new finding — embedded helper classes are ALSO missing
+
+`AlarmIndicator`, `SequenceController`, `NameFilter`, `AverageOf` do **not** exist anywhere in
+the 4.10 CODE archive (confirmed by full-tree grep). They are additional helper classes the
+milestone must reconstruct, and their sizes set the embedded-member offsets — so the
+`masterAlarm@0x39C`/`heatAlarm@0x450`/`stabilityAlarm@0x4C4`/`statusAlarm@0x714` spacing
+DERIVES the AlarmIndicator per-level size (e.g. masterAlarm 33 levels spans 0x39C..0x450 =
+0xB4; heatAlarm 3 levels spans 0x450..0x4C4 = 0x74 — a fixed header + N-level array; solve
+the two equations for header size + per-level stride). Same method sizes AverageOf from
+telemetryFilter[5]@0x7E0..0x81C (0x3C/5 = 0xC each) and SequenceController from the
+legAnimation/bodyAnimation spacing.
+
+### Subsystem roster source availability (VDATA 0xBxx ClassIDs)
+
+- Full .CPP/.HPP in 4.10: **GAUSS, PPC, SENSOR**
+- .TCP partial survivors (authoritative where present): **AMMOBIN, EMITTER, GNRATOR, HEAT,
+  MISSILE, PROJTILE, PROJWEAP**
+- Decomp-only (BT411): MechSubsystem base, gyro, torso, hud, myomers, mechweap, mislanch,
+  powersub, + the rest of the "917 missing functions".
+
+### Revised reconstruction order
+
+1. **Helper classes** (block the layout): AlarmIndicator, SequenceController, NameFilter,
+   AverageOf<Scalar> — sizes solved from the offset-spacing equations above; identify BTVal.
+2. **MechSubsystem base + roster** (GAUSS/PPC/SENSOR + .TCP partials + decomp).
+3. **Finalize MECH.HPP** per the offset map; probe sizeof(Mech)==0x854.
+4. **Mech ctor** (@004a1674): embedded construction + segment-table walk.
+5. Per-frame path (mech2/3/4) + damage/weapon handlers.