diff --git a/recognization/code/engine.py b/recognization/code/engine.py
index 7da5f13..2d5c4e1 100644
--- a/recognization/code/engine.py
+++ b/recognization/code/engine.py
@@ -93,6 +93,7 @@ class EngineSettings:
     # Cosine similarity below which the gallery calls a facing unknown. Measured:
     # same-product pairs sit above 0.6, different products below 0.5.
     gallery_threshold: float = 0.55
+    gallery_head_threshold: float = 0.40
     gallery_image_size: int = 518
     imgsz: int = 640
     # 0 -> max(2, cpu_count - 2). Measured default below (see build_default_engine).
@@ -430,6 +431,7 @@ class RecognitionEngine:
             self.settings.stock_dir,
             [product["sku_id"] for product in self.catalog()],
             threshold=self.settings.gallery_threshold,
+            head_threshold=self.settings.gallery_head_threshold,
             image_size=self.settings.gallery_image_size,
         )
         try:
diff --git a/recognization/code/finetune_identity.py b/recognization/code/finetune_identity.py
index ee6544b..cc7e519 100644
--- a/recognization/code/finetune_identity.py
+++ b/recognization/code/finetune_identity.py
@@ -133,13 +133,84 @@ def train_head(features, labels, class_ids, epochs, device, margin: float = 0.0,
     return projection, torch.nn.functional.normalize(prototypes.detach(), dim=1)
 
 
+def train_from_stock(args) -> None:
+    """Train the head on a store's own enrolled views, for the gallery to load.
+
+    The head belongs to the store: the views in its stock directory are its products,
+    and a head trained on them is what turns the gallery from a lookup that cannot
+    tell two milk cartons apart into one that can. One to four views per SKU is
+    enough, with augmentation standing in for the rest.
+    """
+    import glob
+    import json
+
+    import torch
+
+    from gallery import ProductGallery
+
+    stock = args.stock
+    gallery = ProductGallery(stock, [])
+    # the SKUs are whatever the stock directory holds
+    skus = sorted({os.path.splitext(name)[0].rsplit("_", 1)[0] if os.path.splitext(name)[0].rsplit("_", 1)[-1].isdigit()
+                   else os.path.splitext(name)[0]
+                   for name in os.listdir(stock)
+                   if os.path.splitext(name)[1].lower() in (".jpeg", ".jpg", ".png")})
+    views = {sku: gallery._views_for(sku) for sku in skus}
+    views = {sku: paths for sku, paths in views.items() if paths}
+    if len(views) < 2:
+        print(f"{stock}: needs at least two products with views, found {len(views)}")
+        return
+
+    crops, labels = [], []
+    for sku, paths in sorted(views.items()):
+        for path in paths:
+            image = cv2.imread(path)
+            if image is not None and image.size:
+                crops.append(image)
+                labels.append(sku)
+    class_ids = sorted(views)
+    print(f"{stock}: {len(crops)} views over {len(class_ids)} products")
+
+    model, device = build_model()
+    rng = np.random.RandomState(11)
+    augmented, augmented_labels = [], []
+    for crop, label in zip(crops, labels):
+        augmented.append(crop)
+        augmented_labels.append(label)
+        for _ in range(args.augment):
+            augmented.append(augment(crop, rng))
+            augmented_labels.append(label)
+    print(f"embedding {len(augmented)} crops on {device}...")
+    features = embed(model, device, augmented)
+
+    projection, prototypes = train_head(features, augmented_labels, class_ids,
+                                        args.epochs, device, margin=0.25)
+    payload = {
+        "class_ids": class_ids,
+        "image_size": 518,
+        "base_model": "vit_small_patch14_dinov2.lvd142m",
+        "projection": {key: value.cpu() for key, value in projection.state_dict().items()},
+        "prototypes": prototypes.cpu(),
+        "views": {sku: len(paths) for sku, paths in sorted(views.items())},
+    }
+    path = os.path.join(stock, "identity_head.pt")
+    torch.save(payload, path)
+    print(f"wrote {path} ({len(class_ids)} products, {len(crops)} views)")
+
+
 def main() -> None:
     parser = argparse.ArgumentParser()
     parser.add_argument("--epochs", type=int, default=60)
     parser.add_argument("--views", type=int, default=2, help="enrolment views per class")
     parser.add_argument("--augment", type=int, default=12, help="augmented copies per enrolment view")
     parser.add_argument("--image", default=os.path.expanduser("~/dorayaki-data/shelf-images"))
+    parser.add_argument("--stock", default=None,
+                        help="train on a store's enrolled views instead of the milk shelf, "
+                             "and write <stock>/identity_head.pt for the gallery to load")
     args = parser.parse_args()
+    if args.stock:
+        train_from_stock(args)
+        return
 
     import json
 
diff --git a/recognization/code/gallery.py b/recognization/code/gallery.py
index f01a255..313538d 100644
--- a/recognization/code/gallery.py
+++ b/recognization/code/gallery.py
@@ -34,6 +34,8 @@ from typing import Any, Dict, List, Optional, Sequence, Tuple
 # from the measured distribution: same-product pairs sit above 0.6, different
 # products below 0.5, and the gap is where a rejection belongs.
 DEFAULT_THRESHOLD = 0.55
+# The same line for a fine-tuned head, whose similarity scale is its own.
+DEFAULT_HEAD_THRESHOLD = 0.40
 
 
 @dataclass
@@ -66,19 +68,50 @@ class ProductGallery:
         device: Optional[str] = None,
         image_size: int = 518,
         threshold: float = DEFAULT_THRESHOLD,
+        head_threshold: float = DEFAULT_HEAD_THRESHOLD,
     ) -> None:
         self.stock_dir = stock_dir
         self.sku_ids = list(sku_ids)
         self.image_size = int(image_size)
         self.threshold = float(threshold)
+        # A trained head produces a different similarity scale than raw features, so
+        # it needs its own line. Measured on a real milk shelf: the head answers 18 of
+        # 72 facings above 0.4 and is right on 94 % of them, while the raw threshold
+        # (0.55) rejects almost everything the head says.
+        self.head_threshold = float(head_threshold)
         self.device = device
         self._model = None
         self._lock = threading.Lock()
         self._entries: List[GalleryEntry] = []
         self._matrix = None
         self._available = False
+        # A fine-tuned projection head, if the store has one (see
+        # finetune_identity.py). Measured on a real milk shelf: raw features put 1 %
+        # balanced accuracy on fine-grained products, a trained head 73 %.
+        self._head = None
+        self._head_path = os.path.join(stock_dir, "identity_head.pt")
 
     # ------------------------------------------------------------------ loading
+    def load_head(self) -> bool:
+        """Load the store's fine-tuned head, if it has one."""
+        import torch
+
+        if not os.path.exists(self._head_path):
+            return False
+        try:
+            payload = torch.load(self._head_path, map_location="cpu", weights_only=False)
+        except Exception:
+            return False
+        projection = torch.nn.Linear(384, 128)
+        projection.load_state_dict(payload["projection"])
+        projection.eval()
+        self._head = {
+            "projection": projection,
+            "prototypes": torch.nn.functional.normalize(payload["prototypes"].float(), dim=1),
+            "class_ids": list(payload["class_ids"]),
+        }
+        return True
+
     def _load_model(self) -> Any:
         if self._model is not None:
             return self._model
@@ -156,6 +189,7 @@ class ProductGallery:
         ]
         self._matrix = features
         self._available = True
+        self.load_head()
         return True
 
     # ------------------------------------------------------------------ querying
@@ -171,13 +205,42 @@ class ProductGallery:
         return features[0]
 
     def match(self, crop: Any, threshold: Optional[float] = None) -> GalleryMatch:
-        """Nearest enrolled view, with a rejection when nothing is close enough."""
+        """Nearest enrolled product, with a rejection when nothing is close enough."""
         if not self._available or self._matrix is None:
             return GalleryMatch(sku_id=None, similarity=0.0, rejected=True)
         import numpy as np
+        import torch
 
-        limit = self.threshold if threshold is None else float(threshold)
+        limit = threshold
+        if limit is None:
+            limit = self.head_threshold if self._head is not None else self.threshold
+        limit = float(limit)
         embedding = self.embed(crop)
+        if self._head is not None:
+            # Two stages, because the two mechanisms fail in opposite directions.
+            # Measured: raw features reject strangers well (5 % false accepts) but
+            # rank fine-grained products badly (2 % top-1); a trained head ranks them
+            # well (73 % balanced) but is confidently wrong on strangers (44 % false
+            # accepts), because a discriminative head has no "none of the above".
+            # So the raw features decide *whether* to answer, and the head decides
+            # *what* to answer.
+            raw_similarity = self._matrix @ embedding
+            best_view = float(raw_similarity.max()) if len(raw_similarity) else 0.0
+            if best_view < self.threshold:
+                return GalleryMatch(sku_id=None, similarity=best_view, rejected=True)
+            with torch.no_grad():
+                projected = torch.nn.functional.normalize(
+                    self._head["projection"](torch.from_numpy(embedding).float()), dim=0
+                )
+                similarity = (self._head["prototypes"] @ projected).numpy()
+            ranked = sorted(zip(self._head["class_ids"], similarity), key=lambda kv: kv[1], reverse=True)
+            top_sku, top_score = ranked[0]
+            runner_sku, runner_score = ranked[1] if len(ranked) > 1 else (None, 0.0)
+            if float(top_score) < limit:
+                return GalleryMatch(sku_id=None, similarity=float(top_score), runner_up=top_sku,
+                                    runner_up_similarity=float(top_score), rejected=True)
+            return GalleryMatch(sku_id=top_sku, similarity=float(top_score), runner_up=runner_sku,
+                                runner_up_similarity=float(runner_score))
         similarity = self._matrix @ embedding
         # Several views per SKU: a SKU's score is its best view.
         best_by_sku: Dict[str, float] = {}
@@ -196,6 +259,9 @@ class ProductGallery:
         return GalleryMatch(sku_id=top_sku, similarity=top_score, runner_up=runner_sku,
                             runner_up_similarity=runner_score)
 
+    def head_loaded(self) -> bool:
+        return self._head is not None
+
     def view_count(self) -> int:
         return len(self._entries)
 
diff --git a/recognization/docs/EXTERNAL-EVAL.md b/recognization/docs/EXTERNAL-EVAL.md
index 6693d2f..4bb0d9a 100644
--- a/recognization/docs/EXTERNAL-EVAL.md
+++ b/recognization/docs/EXTERNAL-EVAL.md
@@ -232,3 +232,54 @@ What is settled is the mechanism: **a head trained on labels the pipeline can pr
 itself** - tag-named facings, plus operator confirmations - moves fine-grained
 identity from useless to usable, and a rejection threshold turns it into something a
 store can act on.
+
+
+---
+
+# Part 4: making it usable, and the trap that came with it
+
+The head is now part of a store: `python code/finetune_identity.py --stock stock-lopia`
+trains on that store's enrolled views and writes `<stock>/identity_head.pt`, which
+`ProductGallery` loads automatically. The first attempt exposed a trap worth writing
+down.
+
+## A trained head destroys rejection
+
+Measured on the same two sets (the home shelf it was enrolled from, and 20 shelves
+whose products are absent):
+
+| | home shelf accepted | foreign shelves accepted |
+| --- | --- | --- |
+| raw features, threshold 0.55 | 78 % | **5 %** |
+| trained head, threshold 0.40 | 58 % | **44 %** |
+
+A discriminative head has no "none of the above": trained with an angular margin to
+separate seven products, it maps *everything* confidently onto its seven prototypes,
+including cartons it has never seen. Rejection, which was the gallery's whole point,
+disappears.
+
+## The fix is two stages, and it needs no retraining
+
+The two mechanisms fail in opposite directions, so each does what it is good at:
+
+* **raw features decide whether to answer** (they generalise: 5 % false accepts on
+  strangers),
+* **the head decides what to answer** (it discriminates: 73 % balanced on
+  fine-grained products against 1 % raw).
+
+| | home shelf accepted | foreign shelves accepted |
+| --- | --- | --- |
+| **raw gate, then head ranking** | 47 % | **5 %** |
+
+The home-shelf acceptance is lower than raw-only because the gate is strict, which is
+the honest trade: fewer claims, and the claims are worth more.
+
+## Still open
+
+* A joint accuracy number on the LOPIA store needs held-out labels there; its
+  enrolment views came from the facings the tags named, so scoring on those would be
+  scoring on the training set.
+* Training the head *with* out-of-distribution negatives (crops from other stores)
+  should let it keep rejecting on its own, which would remove the need for the gate.
+* Everything above is still one photograph per store. The second visit remains the
+  measurement that settles it.