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the number refers to the diameter of the bolt whereas in a fitting it refers to the OD of the tube and thereby have different threads. For example, AN6 bolt has a 3/8-24 thread whereas an -6 AN fitting has a 9/16-18 thread.

Originally parts were made compliant to the specification MIL-F-5509, but they are now controlled under SAE AS (Aerospace Standards) specifications AS4841 through AS4843 and AS4875.