Размер дюймовый, а три цифры в маркировке - это код:
Sizing -- Society of Automotive Engineers AS568B is an Aerospace Standard that specifies sizes for O-rings used in sealing applications. An AS568B number is a three-digit suffix that identifies the O-ring's size. Technically, an O-ring size is listed as AS568B-016, AS568B-112, etc., but most people use just the three-digit number. The first digit denotes the O-ring cross section width: 0xx = 1/16-inch, 1xx = 3/32-inch, 2xx = 1/8-inch....
AS568B- Outside Diameter SCUBA Application Seal
214 1-1/4____Cylinder with Large Neck/Valve (3/4 NPS - All Aluminum, most Steel cylinders)
116 15/16____Cylinder with Small Neck/Valve (7/8 UNF - "Genesis" 3500 psi Steel cylinders)
112 11/16____Standard DIN Regulator/Valve (Nearly all DIN regulators)
111 5/8______Small DIN Regulator/Valve (You know if you need this one)
015 11/16____Cylinder Valve Bonnet Nut
014 5/8______Standard Yoke Regulator/K-valve
013 9/16_____Large Low-Pressure Port/Hose (1/2 UNF)
012 1/2______High-Pressure Port/Hose (7/16 UNF) Manifold and Manifold Port Plugs
011 7/16_____Standard Low-Pressure Port/Hose (3/8 UNF)
010 3/8______Low-Pressure Hose/2nd Stage Regulator Cylinder Valve Stem
006 1/4______BC Power Inflator
003 3/16_____High-Pressure Hose/SPG swivel
ссыль, где и внутренний, и внешний диаметр указаны, толщина тора и вид сечения.
http://www.marcorubber.com/sizingchart.htm