In truth, there is no such thing as a chronometer watch. The word 'chronometer' was cheerfully expropriated by the Swiss watch industry to promote the precision, quality and durability of the Swiss product.
So, high quality Omegas that carry the word chronometer are simply lever escapement movements that are of such quality and precision that they meet the criteria established under ISO standard 3159.
The Swiss chronometer timepieces of today are tested for fifteen days, in five positions, at three different temperatures at a Contrôle Officiel Suisse des Chronomètres centre. Each timepiece must at least demonstrate an average daily rate of-4/+6 seconds.
In the heyday of the Omega Constellation, movements were tested by independent Bureaux officiels de contrôle de la marche des montres (B.O.s). The standard required at the time was a mean daily rate in five positions of -1/+10 seconds.
A 'real' chronometer actually describes the escapement of a high precision movement. In effect, the word relates to a highly sensitive and accurate escapement and not the whole movement. Chronometer escapements are of such delicacy and exactitude that they would not be suitable for the rugged wristwatches of today or indeed the 1950s and 60s.
To see what a real chronometer escapement looks like, play the accompanying video.