C-21 means Cargo aircraft model 21 Learjet in Military
Similar to C-21
12N - Horizontal Construction Engineer13 SCVI - 13th South Carolina Volunteer Infantry Regiment
13AD - 13th Airborne Division
13B - Military Occupational Specialty code for Cannon Artilleryman
147FA - 147th Field Artillery Regiment
15MG - 15 Militia Group
17AD - 17th Airborne Division
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