German U-Boat Kreiselsextansen KS-3D 1945 (SEXT51)
This is a WWII-era nautical sextant used on German submarines.
According to collector George Fulford's notes:
This beauty cost a pretty penny. It is a wonderful example of excellent German engineering. This sextant was made late in the war and is in remarkable condition. As you can see the Plath name has been replaced by a three letter code GTL. A rare, one of a kind sextant since it has an internal gyroscope to create the bubble. A fine piece of gear which was ahead of its time, made as a self stabilizing unit for navigating from a rocking U boat or other vessel like the torpedo boats or schnellboote which had trouble getting a steady horizon on a rough ocean.