Key ring measuring app. 1 inch in diameter, fashioned in Sterling Silver, set with the FB 1605 NATO Survival Button Compass, also known as the Francis Barker SAS Survival Button Compass. The tiny brass button compass has a jeweled bearing and is built to full military specifications for survival use. This "brass button" compass is a standard NATO inventory item, NSN 6605 99 522 0223, as specified by the British Army. This Compass is designed to sustain the harshest conditions and during WWII, was even swallowed by military pilots during capture to later be used as an escape compass. It was also sewn into clothes to prevent loss and ensure that its owner always had a compass available. Contemporary.