diff --git a/src/saveload/extended_ver_sl.h b/src/saveload/extended_ver_sl.h
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+++ b/src/saveload/extended_ver_sl.h
@@ -0,0 +1,138 @@
+/* $Id$ */
+
+/*
+ * This file is part of OpenTTD.
+ * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
+ * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/** @file extended_ver_sl.h Functions/types related to handling save/load extended version info. */
+
+#ifndef EXTENDED_VER_SL_H
+#define EXTENDED_VER_SL_H
+
+#include "../core/bitmath_func.hpp"
+
+#include <vector>
+
+enum SaveLoadVersion : uint16;
+
+/**
+ * List of extended features, each feature has its own (16 bit) version
+ */
+enum SlXvFeatureIndex {
+	XSLFI_NULL                          = 0,      ///< Unused value, to indicate that no extended feature test is in use
+	//===================================
+	// COMMON EXTENDED FEATURES
+	//===================================
+	XSLFI_VERSION_LABEL,                          ///< Version label
+	XSLFI_PATCHPACK_MAJOR_VERSION,                ///< Patchpack major version
+	//===================================
+	// PATCHPACK 5.X FEATURES
+	//===================================
+	XSLFI_DAYLENGTH,                              ///< Variable day length
+	XSLFI_STATION_NEWGRF,                         ///< Station names based on industries
+	XSLFI_INFRASTRUCTURE,                         ///< Custom infrastructure base costs
+	XSLFI_RUNNING_COST_MULT,                      ///< Running cost multiplier
+	XSLFI_PLANE_RANGE_MULT,                       ///< Plane range multiplier
+	XSLFI_EXTRA_EXPENSES,                         ///< Additional expense types in the expenses window
+	XSLFI_IMPROVED_BREAKDOWNS,                    ///< Improved breakdowns patch
+	XSLFI_BREAKDOWN_SCALER,                       ///< Additional breakdown chance scaler
+	XSLFI_TOWN_IMPROVEMENTS,                      ///< Configurable bribe risk and town growth factor
+	XSLFI_DILAPIDATION,                           ///< Station dilapidation
+	//===================================
+	XSLFI_SIZE,                                   ///< Total count of features, including null feature
+};
+
+extern uint16 _sl_xv_feature_versions[XSLFI_SIZE];
+
+/**
+ * Operator to use when combining traditional savegame number test with an extended feature version test
+ */
+enum SlXvFeatureTestOperator {
+	XSLFTO_OR                           = 0,      ///< Test if traditional savegame version is in bounds OR extended feature is in version bounds
+	XSLFTO_AND                                    ///< Test if traditional savegame version is in bounds AND extended feature is in version bounds
+};
+
+/**
+ * Structure to describe an extended feature version test, and how it combines with a traditional savegame version test
+ */
+struct SlXvFeatureTest {
+	using TestFunctorPtr = bool (*)(uint16, bool);  ///< Return true if feature present, first parameter is standard savegame version, second is whether standard savegame version is within bounds
+
+	private:
+	uint16 min_version;
+	uint16 max_version;
+	SlXvFeatureIndex feature;
+	SlXvFeatureTestOperator op;
+	TestFunctorPtr functor = nullptr;
+
+	public:
+	SlXvFeatureTest()
+			: min_version(0), max_version(0), feature(XSLFI_NULL), op(XSLFTO_OR) { }
+
+	SlXvFeatureTest(SlXvFeatureTestOperator op_, SlXvFeatureIndex feature_, uint16 min_version_ = 1, uint16 max_version_ = 0xFFFF)
+			: min_version(min_version_), max_version(max_version_), feature(feature_), op(op_) { }
+
+	SlXvFeatureTest(TestFunctorPtr functor_)
+			: min_version(0), max_version(0), feature(XSLFI_NULL), op(XSLFTO_OR), functor(functor_) { }
+
+	bool IsFeaturePresent(SaveLoadVersion savegame_version, SaveLoadVersion savegame_version_from, SaveLoadVersion savegame_version_to) const;
+};
+
+bool SlXvIsFeaturePresent(SlXvFeatureIndex feature, uint16 min_version = 1, uint16 max_version = 0xFFFF);
+
+/**
+ * Returns true if @p feature is missing (i.e. has a version of 0)
+ */
+inline bool SlXvIsFeatureMissing(SlXvFeatureIndex feature)
+{
+	return !SlXvIsFeaturePresent(feature);
+}
+
+const char *SlXvGetFeatureName(SlXvFeatureIndex feature);
+
+/**
+ * sub chunk flags, this is saved as-is
+ * (XSCF_EXTRA_DATA_PRESENT and XSCF_CHUNK_ID_LIST_PRESENT must only be set by the save code, and read by the load code)
+ */
+enum SlxiSubChunkFlags {
+	XSCF_NULL                     = 0,       ///< zero value
+	XSCF_IGNORABLE_UNKNOWN        = 1 << 0,  ///< the loader is free to ignore this without aborting the load if it doesn't know what it is at all
+	XSCF_IGNORABLE_VERSION        = 1 << 1,  ///< the loader is free to ignore this without aborting the load if the version is greater than the maximum that can be loaded
+	XSCF_EXTRA_DATA_PRESENT       = 1 << 2,  ///< extra data field is present, extra data in some sub-chunk/feature specific format, not used for anything yet
+	XSCF_CHUNK_ID_LIST_PRESENT    = 1 << 3,  ///< chunk ID list field is present, list of chunks which this sub-chunk/feature adds to the save game, this can be used to discard the chunks if the feature is unknown
+
+	XSCF_IGNORABLE_ALL            = XSCF_IGNORABLE_UNKNOWN | XSCF_IGNORABLE_VERSION, ///< all "ignorable" flags
+};
+DECLARE_ENUM_AS_BIT_SET(SlxiSubChunkFlags)
+
+struct SlxiSubChunkInfo;
+
+typedef uint32 SlxiSubChunkSaveProc(const SlxiSubChunkInfo *info, bool dry_run);  ///< sub chunk save procedure type, must return length and write no data when dry_run is true
+typedef void SlxiSubChunkLoadProc(const SlxiSubChunkInfo *info, uint32 length);   ///< sub chunk load procedure, must consume length bytes
+
+/** Handlers and description of chunk. */
+struct SlxiSubChunkInfo {
+	SlXvFeatureIndex index;                       ///< feature index, this is saved
+	SlxiSubChunkFlags flags;                      ///< flags, this is saved
+	uint16 save_version;                          ///< version to save
+	uint16 max_version;                           ///< maximum version to accept on load
+	const char *name;                             ///< feature name, this *IS* saved, so must be globally unique
+	SlxiSubChunkSaveProc *save_proc;              ///< save procedure of the sub chunk, this may be nullptr in which case no extra chunk data is saved
+	SlxiSubChunkLoadProc *load_proc;              ///< load procedure of the sub chunk, this may be nullptr in which case the extra chunk data must be missing or of 0 length
+	const char *chunk_list;                       ///< this is a list of chunks that this feature uses, which should be written to the savegame, this must be a comma-seperated list of 4-character IDs, with no spaces, or nullptr
+};
+
+void SlXvResetState();
+
+void SlXvSetCurrentState();
+
+void SlXvCheckSpecialSavegameVersionsA();
+
+void SlXvCheckSpecialSavegameVersionsB();
+
+bool SlXvIsChunkDiscardable(uint32 id);
+
+#endif /* EXTENDED_VER_SL_H */