Editors' Picks Our White Whale Watches

Before there was COSC, a number of independent observatories across Europe were responsible for testing and evaluating the accuracy of mechanical movements and officiating accuracy competitions The observatories of Neuchâtel and Geneva in Switzerland, and Besançon, across the border in neighboring France, were among the best known There was also England's Kew Observatory, affiliated with the Greenwich Observatory, which applied its own highly rigorous methods over a period of 44 days, almost three times the length of a typical chronometer test.

Rolex submitted movements to Kew, a small number of which were cased up in gold in 1954 and outfitted with dials designating their status as observatory chronometers that had earned the "Kew A" Certificate While I know of a single collector who owns one of these rare watches, I've never actually seen one in person. . Source