Hello Rauhbatz,
During WWII there were FOUR AAF's in Kansas that were allocated to Basic Flying Training:
Coffeyville AAF: BT-13, BT-14...…...No Field letter code assigned
Garden City AAF: BT-13...……………..No Field letter code assigned
Independence AAF: BT-13, BT-14...Field Letter code of "S" (prefix)
Strother AAF: BT-13...…………………. Field Letter code of "K"(suffix)
JDV
http://www.fuselagecodes.com