Periscope, Type M6, for Armored Tanks Sherman, Stuart
Here is a fine specimen of the Periscope, Type M6, as used in US WWII-era armored tanks such as the Sherman and Stuart.
From the website The Sherman Tank:
"...there were 12, M6 periscopes in each late model Sherman. Six mounted in in various places, one in the driver’s hatch that rotated, and a fixed one in front of him. The co-driver had the same layout, just on the other side, he used the hatch or fixed periscope to aim the bow machine gun. The loader had had a rotating periscope at their station, and the commander had one in his hatch. There were two complete M6 periscopes in brackets on the turret walls, one near the loader, and the other by the commander. The amazing piece of American tank engineering, the driver’s hood holder, stored four complete M6 periscopes..."
This periscope was mounted by inserting the unit into a bracket and hooked onto a lip (the 'hook' is visible in the photos). The knurled center knob was then tightened to secure it in place. If either of the two glass lenses were shattered during a firefight, the lense could be quickly removed and replaced by turning the flipped-knobs on each side of the periscope nearest the damaged lens, which would release the lens for replacement.
See the photo of the Sherman Tank with the driver's and assistant driver's hatches open, and similar periscopes are seen mounted to the hatch lids.
This periscope appears to be new old stock. The paper card for the adjustment settings is still affixed, all knobs function as specified, the lenses are intact and easily removed.