https://appleinsider.com/articles/17/06/28/apple-ceases-signing-of-ios-1031-after-release-of-ios-1032
Apple ceases signing of iOS 10.3.1 after release of iOS 10.3.2 | AppleInsider
Apple on Tuesday stopped signing code for iOS 10.3.1, halting downloads and installations of the legacy operating system in favor of iOS 10.3.2.
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