The most useful number is the first one. This number appears on Geneva labels. It is the crystal's largest diameter in lignes. Lignes are an obscure system of measure still commonly used to describe the size of watch movements.
In the 18th and 19th centuries, American watch companies used a different system referred to as the Lancashire Gauge, while European companies used lignes.
One lignes is one twelfth of an inch. That inch however is not the familiar English inch, but a Paris inch. A Paris inch is larger at 1.06577 English inches. A lignes therefore works out to 2.25583 millimeters.
- One lignes = 2.25583 mm
- 1/16th lignes = 0.141 mm
A 20 10/16s lignes crystal is 46.5266 mm in diameter.
Watch crystal lignes sizes are always, as far as I know, give in increments of sixteenths. Why 1/16s? A Paris inch is divided into 12 parts called lignes, as we noted. But that system further divides lignes into 12 parts called douziemes. A douziemes is thus 1/144th of a Paris inch. This increment is not precise enough for watch crystals. It was found that a sixteenth of a lignes worked out much better than using twelfths, and so that's the way it went.
Back to the label at hand, the lower number, 3, refers to the height of the crystal. This is an integer value that may range from 2 or 3 up to 8 or 9. The lower the number, the higher the crystal. Although a standard, the scale seems arbitrary and its origins are, to me, unknown.
Here is a second label.
This one just has two numbers. The first number has the sixteenths part, so that is the diameter in lignes.
The middle number on the label above, 24, and the second number on this label, 21 7/8 are a partial mystery.
The first number, 24, is in units of an even more obscure system of measure. These are lunettes. As a system of measure, these units appear to be unique to watch crystals. I know nothing about the origin of these units.
On many other crystal labels with three numbers, that third number will be the diameter in millimeters, usually tenths on millimeters, this 473 would be 47.3 mm. I didn't happen to have a crystal like that handy.