Here's some challenge coins I've picked up over the years.
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48th Communications Squadron; RAF Lakenheath, UK
48th Tactical Fighter Wing Millennium Coin; RAF Lakenheath, UK
92nd Communications Squadron Commander's Coin; Fairchild AFB, WA
92nd Air Refueling Wing Commander's Coin; Fairchild AFB, WA
92 ARW 2002 Annual Awards
First Sergeant's Coin
92nd Security Forces Squadron; Fairchild AFB, WA
Kisling NCO Academy Class 99-2 Flight 8; Kapaun AB, GE
31st Security Forces Squadron; Aviano AB, IT
352nd Operations Support Squadron (Black Knights); RAF Mildenhall, UK
C-135 Airframe Coin
Defense Threat Reduction Agency (DTRA) Coin