[grinditout]

Here is an excerpt from Mcmaster Carr

Dash numbers are a common way to find replacement O-rings without having to measure the width (also referred to as cross section) and ID of an inch-sized O-ring.

Dash numbers organize O-rings into six groups: 000, 100, 200, 300, 400, and 900. O-ring widths are the same within each group, except for some outliers in groups 000 and 900. For example, the 100s all are 0.103" wide. Within each group, the ID increases with the dash number.

The 900s, also known as boss seals, are sized specifically for straight-thread tube fittings. For every increase in dash number, the tube size increases 1/16".

All O-rings with the same dash number have the same dimensions regardless of cross section. Some dash numbers might include a letter or a few additional numbers. These vary based on manufacturer and often refer to the material or hardness of the O-ring.