SAE AS568 is the U.S. standard dash-number system for O-rings, and the three digits decode systematically. The first digit selects the cross-section series: −0xx rings have a 0.070 in section, −1xx are 0.103 in, −2xx are 0.139 in, −3xx are 0.210 in, and −4xx are 0.275 in. The remaining digits step through inside diameters, mostly in 1/16 in nominal increments. An O-ring is specified by ID and cross-section, never by OD — the outside diameter is just ID + 2 × CS, computed rather than standardized — and the actual ID always runs a few thousandths under the fractional nominal (a "1 inch" −214 measures 0.984 in) so the ring stretches slightly onto its groove.
The dash number is also the handle gland design hangs on: pick the dash and the Parker handbook’s design charts give the groove depth, width, radius, and clearance for its cross-section family, with the dash dimensions themselves the same in every application. To identify an unknown ring, measure the cross-section first — it tells you the series — then measure the ID and find the nearest tabulated row; a ring that falls between rows is likely metric or a special, the common metric sections (1.78, 2.62, 3.53, 5.33, 7.00 mm) being near-equivalents of the five inch series rather than members of them.