Modex

A modex is part of the Aircraft Visual Identification System* which consists of either one or two letters used by Dept of Navy (USN and USMC) aircraft to identify aircraft unit assignment and side numbers.

Use
The first letter of a modex indicates whether the organization (wing, group, squadron, station) owning the aircraft originated with Marine/Naval Air Forces Atlantic (A through M), Marine/Naval Air Forces Pacific (N through Z) or in the case of CNATRA a single letter (A through G) for the Training Wing.