The story of pre-moon Speedmasters such as this is well documented on the web, NASA purchased in the early 60’s and put through a very rigorous series of tests several watches before choosing the Speedmaster to be used on all space missions. This particular Speedmaster however was used on a different type of mission - a polar expedition. On April 20, 1968 four explorers led by Ralph Plaisted arrived at the north pole, with their Speedmasters on their wrists, after a 44 day voyage with a daily average temperature of -52 Degrees. Read about the expedition: http://www.qsl.net/kg0yh/plaisted.htm
This watch is engraved on the back Plaisted Polar Expedition 1968. I presume that it belonged to one of the explorers or their support team. Watch is in excellent working condition. Measures 40 x 47 mm (not including crown and pushers) and is fit with a modern (ref 1498/840) Omega bracelet.