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Demolition Firing Devices Mk 12 , Mk 13, and Mk 15 Mod 0.

Text copied from OP1664 vol2 :

These devices are clockwork mechanisms designed to fire demolition charges with a time delay, adjustable—depending upon the model—from five seconds to eleven hours. They are designed for firing either activator-well, or detonating-cord charges.

Photo B. Bosman.

Setting and actuation: ,The Mk 12 may be set from,5 to 90 seconds delay; Mk 13, from 1 minute to 1 hour delay; Mk 15, from 1/4 to 11 hours delay. Because the Mk 15 is as much as 30% inaccurate for settings beneath one hour, these calibrations are coated with red paint as a warning.
To set, turn the head of the device until the desired time delay is opposite the line above “Delay in Hours.” Do not try to turn past the stop pin.
To actuate, pull out the arming ring and the cotter pin attached to it. Make sure there is enough space around the arming pin to allow it to move 3/8 inch outward, After the device is actuated, it cannot be stopped.

Operation: When the cotter pin is removed, the spring-loaded arming pin moves out of the notch in the timing disc, The timing disc, driven by a pre-wound clock movement, can then revolve until the notch meets the timing-disc lever. The lever position andi therefore, the elapsed time, is changed by rotating the head to the desired time setting. A spring-loaded firing pin, positively locked in place by a series of cams and levers, is released when the timingdisc lever enters the notch. The firing pin penetrates the waterproof diaphragm and fires the percussion cap. The flash of flame which results will detonate the Special Non-electric Cap or the Detonating Cord Initiator Mk 2 Mod 0,

Remarks:. Tabs have been placed under the screws fastening the head to the body, to make possible identification by feel in the dark, as follows: Mk 12—no tab; Mk 13—one tab; Mk 15—two tabs.
This device can also be used in booby traps as a trip-wire firing device. The trip wire should be secured to the arming ring and the firing device firmly fastened in place.