(I re-uploaded this to fix some audio issues)
As World War One stagnated into trench warfare, snipers and machine guns quickly proliferated, and exposure above the parapet of one's trench could be extremely hazardous. This leaves one with the question of, how to shoot back without risking a bullet?
One answer that was devised was to mount a rifle to a periscope. That way the rifle could be lifted up to get a clear shot at the enemy trenches while the shooter remained safely out of sight using mirrors to see his sights and a length of wire to pull his trigger. While all the major powers in the war developed devices like this, the one we are looking at today is German. It's simple, but effective.
In addition to the trench periscope stock, this rifle has also been fitted with a couple other WWI modifications. It has clamp-on luminous sights to allow more accurate shooting at night, and also an extended 25-round fixed magazine to give the shooter much more ammo to fire before needing to reload. The rifle also comes with another neat accessory, which was not fitted for the video.
This particular trench stock is a factory-made item, but does not include a mechanism to cycle the bolt - a shooter would have to bring the rifle back down after each shot. Some more complex versions were made which included articulated levers for cycling the bolt from below. The range of trench stocks also goes the other direction, and includes plenty of examples that were handmade in the trenches by individual soldiers...where necessity was the mother of invention. However, trench stocks of any type are quite rare to find, and they made very clumsy souvenirs for troops coming home.