German trench retracting periscope
Periscopes were ones of the most popular devices in trench warfare, commonly used by fighting armies. They were used to observe the enemy positions without leaning out from own trench.
The periscope presented here is probably a custom German periscope. Its simple construction and materials used suggest that it was made during the war by a handyman soldier. It is retractable from both sides. Does not have any markings.