A Smiths 8 day ships 'Zig-zag' clock in full working order with switch, alarm and key dated 1943.
Unusually it has all the original electrical contacts and two knobs at the back to connect.
I wouldn't have thought many examples like this exist.
 
During the 2nd World war Anti-submarine experts diagrammed zig-zag routes and assigned them to convoys, each assembled ship receiving the same detailed itinerary. Following the precise routes took excellent seamanship and the help of a device called a Zig-Zag Clock. 
Each ship in the convoy had the same clock, synchronized with the flagship, and attached to an alarm system in the bridge. It looks like an ordinary clock except for the little metal pegs or clips around the rim that could be moved into different positions. When the minute hand swept past a peg, the alarm sounded, indicating both a speed and course change. Ten to twenty minutes later, it would sound again for another correction. And on it went for days at sea, each ship’s bridge buzzing or ringing in harmony to keep the convoy moving together. 
The key was to produce as irregular and unpredictable zig-zag pattern as possible. Course corrections were set at angles between twenty and forty degrees, the time intervals between ten and twenty minutes, and the entire pattern was changed every forty to eighty minutes. Light, wind, weather, and sea states were all taken into account, and convoys could implement changes to the original zig-zag pattern when flagging conditions allowed.
 
Clock measures 16cm diameter x 9cm high.
Wood backing measures 46cm high x 28.5cm wide x 5cm deep.