1 French line (also referred as Paris line) = 2.2558 mm
Before 1920's, Omega movement sizes were expressed exclusively in French lignes, which gave the name of the caliber. It is not unusual, since the French and Swiss watch manufacturers, always preferred the Paris line to the metric system, when referring to movement sizes. Although this was the first Omega movement reference system, the French line remained in use, even after the movement diameter in metric system indicated the caliber abbreviation (ie. Cal. 35.5L stands for a 35.5 mm movement diameter), throughout 1940's.
Either if it was a Lépine or a Savonette movement, Omega used a practical and easy to remember design. So easy that even a century after, pocket watches manufactured in the French line era, regardless the size, are hard to differentiate. And one of the reasons is that Omega caliber size and quality abbreviation was rarely represented anywhere on the movement.
By the end of the 19th century, Omega was able to manufacture a wide range of qualities, from A to D, in different sizes. In the mare of movements, one organ could make a difference between caliber qualities: the regulator.
In short, the regulator is the adjustable device in the movement, responsible for making the watch run faster or slower, otherwise used to regulate the accuracy.
In all fairness, Chronometer class movements do not qualify for this standard just for having a fancy regulator organ, but it is safe to assume that Chronometers only carry top of the class regulators for fine adjustments and ultra high precision.
Not long ago, I was briefly explaining what a Chronometer class movement means. Even a century ago, each manufactured chronometer was unique, not just by its serial number, but also by performances. After a long period of tests, such movements were provided with a "Bulletin de Marche" from one of the famous Swiss Observatories.
The chart below represents the regulator and balance cock variation during the French line era. Regulators displayed, except the 30" 8 DL, are from Lépine 19" ligne calibers. However, the balance cock and the regulator styles can be observed in almost all movement sizes.
With the introduction of the new movement diameter caliber abbreviation, and later with the 3 digit caliber code, Omega movements were finally easier to differentiate, thus the later ones will not make the subject of this study.