The Vickers Tank Periscope MK.IV, invented by Polish engineer Rudolf Gundlach, was first patented in 1936 as Gundlach Peryskop obrotowy. It was the first device to allow the tank commander to have a 360-degree view from his turret, with a single periscope. By rotating the periscope and allowing the tank commander to look backwards through the second eyepiece, he no longer had to change position to look behind the turret. Early tanks had small turrets and fixed seating, without an independently rotating cupola, and so the commander wasn't easily able to move himself to another rear-facing periscope.
The design was first used in the Polish 7-TP light tank. Shortly before the war it was given to the British and was used in most tanks of WWII, including the British Crusader, Churchill, Valentine, and Cromwell and the American Sherman. The design was later copied and used extensively in tanks of the USSR (including the T-34 and T-70) and Germany.
It was first implemented in TKS and 7TP Polish tanks. As a part of Polish-British pre-war military cooperation, the patent was sold to Vickers-Armstrong. It was produced as the Vickers Tank Periscope MK.IV (pictured), and built into all British tanks (Crusader, Churchill, Valentine, Cromwell). After the fall of Poland, Germany, USSR and Romania captured some equipment, allowing them to copy the invention. In USSR the Gundlach periscope was known as MK-4 (harking to the British designation, as Russian sources openly confirm that it was copied from samples acquired with British-supplied tanks) and implemented in all tanks (including the T-34 and T-70). All Axis tanks and APC (including tanks of Italy, Romania, Hungary, Finland and Japan) were equipped or retro-fitted with this periscope till 1941. Later technology was transferred to USA and as a periscope M6 implemented in all US tanks (M3/M5 Stuart, M4 Sherman and others). At the end of WWII this technology was adopted throughout the world.