C-21 means Cargo aircraft model 21 Learjet in Military
Similar to C-21
63B - Light Wheeled Vehicle Mechanic63Y - Track Vehicle Mechanic Mos
643ASG - 643rd Area Support Group
645TD - 645th Tank Destroyer Battalion
66 TAC - 66th Tatical Aviation Command
67Y - AH-1 Attack Helicopter Repairer MOS
7 WVI - 7th West Virginia Infantry Regiment
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