A Very Sound and Good Condition World War II British Tank Site / Periscope-

Tank & AFV Periscope TD31917. War Dept stampings and maker markings. Likely manufactured 1930's. Perfect prisms. Their locationing can be seen in both the tank photographs, fitted to the two hatch lids in front of the turretsTanks and armoured vehicles use periscopes: they enable drivers, tank commanders, and other vehicle occupants to inspect their situation through the vehicle roof. Prior to periscopes, direct vision slits were cut in the armour for occupants to see out. Periscopes permit view outside of the vehicle without needing to cut these weaker vision openings in the front and side armour, better protecting the vehicle and occupants.A protectoscope is a related periscopic vision device designed to provide a window in armoured plate, similar to a direct vision slit. A compact periscope inside the protectoscope allows the vision slit to be blanked off with spaced armoured plate. This prevents a potential ingress point for small arms fire, with only a small difference in vision height, but still requires the armour to be cut.In the context of armoured fighting vehicles, such as tanks, a periscopic vision device may also be referred to as an episcope. In this context a periscope refers to a device that can rotate to provide a wider field of view (or is fixed into an assembly that can), while an episcope is fixed into position.Periscopes may also be referred to by slang, e.g. 'shufti-scope'.