diff --git a/requirements-swh.txt b/requirements-swh.txt
--- a/requirements-swh.txt
+++ b/requirements-swh.txt
@@ -1,4 +1,4 @@
-swh.model < 0.4.0
-swh.storage >= 0.9.3
+swh.model >= 0.4.0
+swh.storage >= 0.10.0
 swh.scheduler >= 0.0.39
 swh.loader.core >= 0.5.2
diff --git a/swh/loader/mercurial/loader.py b/swh/loader/mercurial/loader.py
--- a/swh/loader/mercurial/loader.py
+++ b/swh/loader/mercurial/loader.py
@@ -470,17 +470,21 @@
             else:
                 directory_id = self.mnode_to_tree_id[commit["manifest"]]
 
-            extra_meta = []
+            extra_headers = [
+                (
+                    b"time_offset_seconds",
+                    str(commit["time_offset_seconds"]).encode("utf-8"),
+                )
+            ]
             extra = commit.get("extra")
             if extra:
                 for e in extra.split(b"\x00"):
                     k, v = e.split(b":", 1)
-                    k = k.decode("utf-8")
                     # transplant_source stores binary reference to a changeset
                     # prefer to dump hexadecimal one in the revision metadata
-                    if k == "transplant_source":
+                    if k == b"transplant_source":
                         v = hash_to_bytehex(v)
-                    extra_meta.append([k, v])
+                    extra_headers.append((k, v))
 
             parents = []
             p1 = self.node_2_rev.get(header["p1"])
@@ -498,16 +502,8 @@
                 type=RevisionType.MERCURIAL,
                 directory=directory_id,
                 message=commit["message"],
-                metadata={
-                    "node": hash_to_hex(header["node"]),
-                    "extra_headers": [
-                        [
-                            "time_offset_seconds",
-                            str(commit["time_offset_seconds"]).encode("utf-8"),
-                        ],
-                    ]
-                    + extra_meta,
-                },
+                metadata={"node": hash_to_hex(header["node"]),},
+                extra_headers=tuple(extra_headers),
                 synthetic=False,
                 parents=tuple(parents),
             )
diff --git a/swh/loader/mercurial/tests/test_loader.py b/swh/loader/mercurial/tests/test_loader.py
--- a/swh/loader/mercurial/tests/test_loader.py
+++ b/swh/loader/mercurial/tests/test_loader.py
@@ -236,8 +236,8 @@
     transplant_sources = set()
     for rev in loader.storage.revision_log(revisions):
         hg_changesets.add(rev["metadata"]["node"])
-        for k, v in rev["metadata"]["extra_headers"]:
-            if k == "transplant_source":
+        for k, v in rev["extra_headers"]:
+            if k == b"transplant_source":
                 transplant_sources.add(v.decode("ascii"))
 
     # check extracted data are valid