Knox County, Nebraska / U.S. Department of Agriculture, Production and Marketing Administration.United States. Department of Agriculture. Production and Marketing Administration[1951?]"Aero Service Corp., Philadelphia PA.""Flying completed 7-20-51. Index copied 9-7-51.""Symbol BNE USDA 1233-51, item no. 1, scale 1:20,000."Photomosaic index.cartographic