diff --git a/include/pybind11/detail/smart_holder_poc.h b/include/pybind11/detail/smart_holder_poc.h
index d0a8ce368..6ec1d1eff 100644
--- a/include/pybind11/detail/smart_holder_poc.h
+++ b/include/pybind11/detail/smart_holder_poc.h
@@ -1,3 +1,7 @@
+// Copyright (c) 2020-2021 The Pybind Development Team.
+// All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
 /* Proof-of-Concept for smart pointer interoperability.
 
 High-level aspects:
diff --git a/include/pybind11/detail/smart_holder_type_casters.h b/include/pybind11/detail/smart_holder_type_casters.h
index ff517be18..ff54c9455 100644
--- a/include/pybind11/detail/smart_holder_type_casters.h
+++ b/include/pybind11/detail/smart_holder_type_casters.h
@@ -1,3 +1,7 @@
+// Copyright (c) 2020-2021 The Pybind Development Team.
+// All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
 #pragma once
 
 #include "../cast.h"
diff --git a/include/pybind11/smart_holder.h b/include/pybind11/smart_holder.h
index fccc45ff1..0f9f6f4c0 100644
--- a/include/pybind11/smart_holder.h
+++ b/include/pybind11/smart_holder.h
@@ -1,3 +1,7 @@
+// Copyright (c) 2021 The Pybind Development Team.
+// All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
 #pragma once
 
 #include "detail/smart_holder_type_casters.h"