An overall view. (Photo courtesy of Synder's Treasures)
This item is a World War I German Fernsprechschlüssel (Telephone Code Key) or Schlüsselschieber (Cipher Slide Rule). It's an early tactical code device, used to encode 200 commonly used words and terms into numeric codes. The operation is fairly simple: the arrow on the sliding portion of the device is aligned with the location indicated for the hour of the day, or day of the month. The device on the photo is keyed for day of the month, and the code is aligned with the words and transcribed into the message.
On the slider, the numbers could range from 1 to 200, ordered, with a transposition and break partially through the numbering scheme for better security. All the terms were arranged in (more or less) alphabetical order, with the more routine terms in ink and the more temporary terms in pencil on the fixed portions of the device. Both sides of the device were used.
A closeup of the lettering on the device. (Photo courtesy of Synder's Treasures)
This style and type of "trench code" system was known as "carnets" and is discussed on pages 314 and 315 of Kahn's book "The Codebreakers: The Story of Secret Writing". Devices of this type were used by both the French and the Germans in 1916, but is was very quickly superseded by more secure communication ciphers.
The device in the photo has three of the four fixed pages mostly intact, with the last page mostly absent,but the slide is virtually intact. If there was potential for the device to fall into enemy hands, it was easy to destroy or burn because it was made from very thin wood.
Overall dimensions for this device are 5.5 by 13 inches, and the slide is around 15 inches long. In practice, the slide is designed to only move about 2 inches to align the arrow to the proper date and optionally hour.
(Text courtesy of Snyder's Treasures. This item was put up for sale on E-bay).