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.
Bosley regulator, stepped balance cock:
Introduced in 1894, this assembly has a few distinctive features: a stepped balance cock, a L shape Bosley regulator with the curb pins at 9, changing the stud regular position. The balance wheel is fitted with a flat hairspring.
Along with the above regulator, these were the first two options offered to the market. What marks a difference between the two, is the Bosley regulator with the curb pins at 11, and the stud on the balance cock extremity, at 9. The balance wheel is fitted with a Breguet hairspring, instead of flat.
Goliath - Bosley regulator, stepped balance cock:
The 30" lig. 8 day caliber was available since 1894, showing a nicely decorated balance cock and a straight Bosley regulator. Even though it was followed by the 27" lig. 8 day caliber in 1896, there is no style difference between the two. An interesting aspect is that, during the French line era, the 27" lig. caliber was the only Omega movement besides the ladies miniature calibers, also available in 17 jewels quality, but with a simple Bosley regulator.
Bosley regulator, flat balance cock:
Flat balance cock, L shape Bosley regulator with the curb pins at 9, changing the stud regular position. The balance wheel is fitted with a flat hairspring. There is no technical difference between the flat an the stepped balance cock. Widely available since early 1900's.
Flat balance cock, Bosley regulator with the curb pins at 11, and the stud on the balance cock extremity, at 9. The balance wheel is fitted with a Breguet hairspring, instead of flat.
Although the stepped balance cock has been gradually replaced by the flat balance cock version, it is not unusual to see them delivered up to 1910's.
Labrador - Bosley regulator, flat balance cock:
This regulator can only be found in the Labrador 3 quarter plate calibers and has a very distinctive pallet bridge. So far, these were observed only in B quality movements. The curb pins are at 12, the regulator index thickness and size is slightly different too, and so is the balance cap jewel setting. The balance wheel is fitted with a Breguet hairspring.
Reed's or Swan Neck regulator, flat balance cock:
The Swan Neck is actually an improved Bosley regulator, assigned to George Reed, and can be observed with almost any reputable manufacture of 19th or 20th century. Omega pocket watches carrying this device were available since 1894, but will not show any patent on the main plate, sign that there has been no contribution to Reed's design.
Star regulator, flat balance cock:
In the French line era, Omega's real contribution, was the Star regulator, for which brevet no. was granted in 1904. A particular feature is that each movement fitted with this device, will show the patent no. 31.050 engraved on the main plate, on the dial side.
Wilmot's or Snail regulator, flat balance cock:
The Snail regulator was assigned to Francois Wilmot of New York, long before Omega, in 1870's. Omega pocket watches featuring this regulator were available since 1894, and were offered with a chronometer class bulletin, certifying a high level of accuracy. Although the Snail regulator found its purpose even in the DR and DDR (later Verybest) grades, not a single movement will show a regulator patent no. on the main plate, sign that there has been no addition to Wilmot's design.