This is a really interesting artifact of the First World War that I found in a collection and wanted to share (since the owner, understandably, wouldn't part with it!). All the major powers in the Great War set up aerial training schools to teach pilots and observer/gunners how to use their guns without having to be up in the air. These often included mocked-up cockpits and gun mounts which could be towed to enable practice while moving.

The Vickers Gun we are looking at today began its life as an aircraft gun, with the water jacket cut away to allow air cooling and reduce weight. It appears that a French training base needed a training gun with an intact water jacket for extended firing (the air cooling doesn't work so well when the gun is essentially static on the ground). Unable to procure a new standard ground Vickers, an armorer spent a significant amount of time re-sealing the jacket on an aircraft gun instead. A major clue to its purpose is the French-language tag riveted to the back of the jacket warning users to ensure that the oscillating arm doesn't hit the jacket - this indicates that it was set up with a firing mechanism to duplicate the synchronizers used on aircraft.

Unfortunately, the gun was disassembled at some point and all the detachable parts removed, along with the right-hand side plate of the receiver (the part that is legally a machine gun in the US). However, the jacket and remnants of the receiver remain a really interesting artifact, to me at least...