diff --git a/mercurial/cext/revlog.c b/mercurial/cext/revlog.c
--- a/mercurial/cext/revlog.c
+++ b/mercurial/cext/revlog.c
@@ -343,18 +343,28 @@
 	}
 }
 /*
+ * Pack header information in binary
+ */
+static PyObject *index_pack_header(indexObject *self, PyObject *args)
+{
+	int header;
+	char out[4];
+	if (!PyArg_ParseTuple(args, "I", &header)) {
+		return NULL;
+	}
+	putbe32(header, out);
+	return PyBytes_FromStringAndSize(out, 4);
+}
+/*
  * Return the raw binary string representing a revision
  */
-static PyObject *index_entry_binary(indexObject *self, PyObject *args)
+static PyObject *index_entry_binary(indexObject *self, PyObject *value)
 {
 	long rev;
-	int header;
 	const char *data;
-	char entry[v2_hdrsize];
-
 	Py_ssize_t length = index_length(self);
 
-	if (!PyArg_ParseTuple(args, "lI", &rev, &header)) {
+	if (!pylong_to_long(value, &rev)) {
 		return NULL;
 	}
 	if (rev < 0 || rev >= length) {
@@ -367,10 +377,8 @@
 	if (data == NULL)
 		return NULL;
 	if (rev == 0) {
-		/* put the header at the start of the first entry */
-		memcpy(entry, data, self->hdrsize);
-		putbe32(header, entry);
-		return PyBytes_FromStringAndSize(entry, self->hdrsize);
+		/* the header is eating the start of the first entry */
+		return PyBytes_FromStringAndSize(data + 4, self->hdrsize - 4);
 	}
 	return PyBytes_FromStringAndSize(data, self->hdrsize);
 }
@@ -2891,8 +2899,10 @@
     {"shortest", (PyCFunction)index_shortest, METH_VARARGS,
      "find length of shortest hex nodeid of a binary ID"},
     {"stats", (PyCFunction)index_stats, METH_NOARGS, "stats for the index"},
-    {"entry_binary", (PyCFunction)index_entry_binary, METH_VARARGS,
+    {"entry_binary", (PyCFunction)index_entry_binary, METH_O,
      "return an entry in binary form"},
+    {"pack_header", (PyCFunction)index_pack_header, METH_VARARGS,
+     "pack the revlog header information into binary"},
     {NULL} /* Sentinel */
 };
 
diff --git a/mercurial/pure/parsers.py b/mercurial/pure/parsers.py
--- a/mercurial/pure/parsers.py
+++ b/mercurial/pure/parsers.py
@@ -127,14 +127,17 @@
             r = (offset_type(0, gettype(r[0])),) + r[1:]
         return r
 
-    def entry_binary(self, rev, header):
+    def pack_header(self, header):
+        """pack header information as binary"""
+        v_fmt = revlog_constants.INDEX_HEADER
+        return v_fmt.pack(header)
+
+    def entry_binary(self, rev):
         """return the raw binary string representing a revision"""
         entry = self[rev]
         p = revlog_constants.INDEX_ENTRY_V1.pack(*entry)
         if rev == 0:
-            v_fmt = revlog_constants.INDEX_HEADER
-            v_bin = v_fmt.pack(header)
-            p = v_bin + p[v_fmt.size :]
+            p = p[revlog_constants.INDEX_HEADER.size :]
         return p
 
 
@@ -286,14 +289,12 @@
             msg = b"cannot rewrite entries outside of this transaction"
             raise KeyError(msg)
 
-    def entry_binary(self, rev, header):
+    def entry_binary(self, rev):
         """return the raw binary string representing a revision"""
         entry = self[rev]
         p = revlog_constants.INDEX_ENTRY_V2.pack(*entry)
         if rev == 0:
-            v_fmt = revlog_constants.INDEX_HEADER
-            v_bin = v_fmt.pack(header)
-            p = v_bin + p[v_fmt.size :]
+            p = p[revlog_constants.INDEX_HEADER.size :]
         return p
 
 
diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -266,7 +266,7 @@
             return (0, 0, 0, -1, -1, -1, -1, sha1nodeconstants.nullid)
         return list.__getitem__(self, i)
 
-    def entry_binary(self, rev, header):
+    def entry_binary(self, rev):
         """return the raw binary string representing a revision"""
         entry = self[rev]
         if gettype(entry[0]):
@@ -284,6 +284,10 @@
         )
         return INDEX_ENTRY_V0.pack(*e2)
 
+    def pack_header(self, header):
+        """Pack header information in binary"""
+        return b''
+
 
 def parse_index_v0(data, inline):
     s = INDEX_ENTRY_V0.size
@@ -2041,7 +2045,10 @@
             self.version &= ~FLAG_INLINE_DATA
             self._inline = False
             for i in self:
-                e = self.index.entry_binary(i, self.version)
+                e = self.index.entry_binary(i)
+                if i == 0:
+                    header = self.index.pack_header(self.version)
+                    e = header + e
                 fp.write(e)
 
             # the temp file replace the real index when we exit the context
@@ -2363,7 +2370,10 @@
             e = e[:8]
 
         self.index.append(e)
-        entry = self.index.entry_binary(curr, self.version)
+        entry = self.index.entry_binary(curr)
+        if curr == 0:
+            header = self.index.pack_header(self.version)
+            entry = header + entry
         self._writeentry(
             transaction,
             ifh,
@@ -3216,5 +3226,8 @@
             for i, entry in enumerate(new_entries):
                 rev = startrev + i
                 self.index.replace_sidedata_info(rev, entry[8], entry[9])
-                packed = self.index.entry_binary(rev, self.version)
+                packed = self.index.entry_binary(rev)
+                if rev == 0:
+                    header = self.index.pack_header(self.version)
+                    packed = header + packed
                 fp.write(packed)
diff --git a/rust/hg-cpython/src/revlog.rs b/rust/hg-cpython/src/revlog.rs
--- a/rust/hg-cpython/src/revlog.rs
+++ b/rust/hg-cpython/src/revlog.rs
@@ -177,6 +177,11 @@
         self.call_cindex(py, "entry_binary", args, kw)
     }
 
+    /// return a binary packed version of the header
+    def pack_header(&self, *args, **kw) -> PyResult<PyObject> {
+        self.call_cindex(py, "pack_header", args, kw)
+    }
+
     /// get an index entry
     def get(&self, *args, **kw) -> PyResult<PyObject> {
         self.call_cindex(py, "get", args, kw)