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The sign accompanying the Vikers 4-inch submarine gun. It reads

Vikers 4-inch Bore
Submarine Gun
WWI (1914)

The French-language portion of the sign notes that it's also known as a Vickers Mark IX 100mm gun.

Google doesn't seem to think that there's any such thing as a "Vikers" gun, instead suggesting "Vickers" (although there doesn't seem to be any information about Vickers Mark IX 4-inch gun, either). The closest thing that Google suggested for "Mark IX gun" was the BL 4-inch Mk IX naval gun, but that doesn't really look like this gun, there's no mention of it being a submarine gun, and it wasn't introduced until 1917.