Flights contracted by USDA use a three letter identifying code for the county photographed.  The flight ID is the three letter county code followed by the year flown.  Below is a map showing USDA county codes for California.  Not all counties were given a code.Map showing California USDA county codes

Code County Code County
AAW Glenn AXN San Diego
AAX Butte BTM Santa Barbara
AAY Colusa BUT Alameda
AAZ Sutter BUU Contra Costa
ABA Yuba BUV Lassen
ABB Yolo BUW Modoc
ABC Sacramento BUX San Benito
ABD San Joaquin BUY  Shasta
ABE Stanislaus CIV  Santa Clara
ABF Merced  CJA  Santa Cruz
ABH Madera  CSH  Sonoma
ABI Fresno  CSI  Napa
ABJ Kings  CVL  Humboldt
ABK Tulare  CVM  Lake
ABL Kern  CVN  Mendocino
ABM Placer  DDB  San Mateo
ABN Imperial  DDC  Siskiyou
ABO Solano  DRH  Marin
AXC Tehama  DSC  El Dorado
AXH San Luis Obispo  DYD  Plumas
AXI Ventura  DYD  Sierra
AXJ Los Angeles  EGL  Inyo
AXK Orange  EJC  Mariposa
AXL San Bernardino  ELW  Plumas
AXM Riverside    

 

Information from: Code symbols: identification of aerial photography. Aerial Photography Division, Agricultural Stabilization and Conservation Service, United States Department of Agriculture. Washington, D.C. 1969. available in MIL Reference: TR810 .C63 1969