To run off on a tangent ... Per Postal Bulletin #18457, dated 7/1/1942, the Camp Atterbury post office opened on July 15, 1942 as a classified branch of Columbus, IN. Here is a first day of operation cover:

The Camp had a machine cancel in operation by late September 1942. Many tens of thousands of troops went through Camp Atterbury. My father was assigned there briefly in 1944 and passed through again in 1946 when discharged.
One of the few WWII-era buildings yet standing in Camp is the chapel built by the Italian POWs. The Camp has a very nice military museum for anyone traveling in the area.