Ordinary wristwatches could not always be relied upon to withstand the opening shock and landing impact stresses imposed upon them during a Second World War military parachute descent. By 1944 a watch specifically designed for the rigours of airborne work had been developed. Using existing stocks of Longines calibre 12.68N movements, which did not possess any shockproofing qualities, the research and development establishment of the War Department had constructed a special case to protect them. Made of brass nickel-plated with a satin finish, the waterproof case was made to a larger diameter than that of the movement; this allowed any violent shock to dissipate through the case rather than be transferred directly to the vulnerable movement. Other improvements include a rimmed glass which was inserted through the back of the watch and then secured by an “O” ring tightly screwed up behind it, eliminating any danger of the glass being knocked out of the case. Reinforced strap lugs were also provided. The screw back is engraved with the Company Ordnance Supply Depot (C.O.S.D.) abbreviation and the store number “2131” (Paratrooper’s Wristwatch); “2340” is the watch’s serial number.