Longines, Ultra-Chron, "High-Beat Collection", wristwatch, 36.5 mm.

Longines cal 431 – “ … the most accurate watch in the world”
Debuting in 1967, the Ultra-Chron was built to celebrate Longines’ centenary, 100 years since its production facility was established in an area of St Imier, Switzerland, known as Les Longines, or ‘the long meadows’. However, the origins of Longines high beat movement development go back to experimentation with high frequency escapements in stopwatches as far back as 1914. These had 36,000 vph movements and were used as timers in the Olympic games of the era being able to time results to one tenth of a second. Later, in1959 with the production of a limited number of observatory chronometers with a new caliber L360 movements, Longines experimented extensively with new materials in the pursuit of extreme timekeeping accuracy.

Development of the cal 431 movement for the introduction of the Ultra-Chron (an abbreviation of ultra-chronometer) probably began in 1965 and was built entirely in-house, but did utilize the Clinergic-21 Swiss-anchor escapement with 21 teeth on the steel escape wheel compared to a standard 15 teeth for conventional movements. Longines was probably the first to use dry-lubrication of fast-moving components with a molybdenum bisulphide lubricant that solved the problems of rapid wear on highly strained high-beat movement components.

Over the nine years of Ultra-Chron production from 1967 to 1976 Longines used four variations of their high-beat movement: Cal. 430 (center seconds), Cal. 431 (center seconds with date), Cal. 432 (hours and minutes only), Cal. 433 (date only). These watches were COSC chronometer-certified when COSC was introduced in 1973. Like Girard-Perregaux, Longines also offered manual wind high-beat movements which featured a hacking second hand (the Cal. 6952 with date and Cal. 6972 with day and date).

At least from 1973 onward, Ultra-Chron watches were guaranteed accurate to within one minute per month, or plus/minus two seconds per day, and were advertised as “the world’s most accurate watch”.