What are the technical differences between A1387 and A1431 iPhones
China around the iPhone 4s and 5 used WAPI variant of wifi and a country specific CDMA network. The A1431 is tailored to those differences. 


http://www.everymac.com/systems/apple/iphone/specs/apple-iphone-4s-gsm-china-unicom-a1431-specs.html



  The China-only iPhone 4S still is believed to be a "world phone," supporting both GSM and CDMA networks (UMTS/HSDPA/HSUPA (850, 900, 1900, 2100 MHz); GSM/EDGE (850, 900, 1800, 1900 MHz), and CDMA EV-DO Rev. A (800, 1900 MHz), and the "antenna automatically switches between send and receive."