watchOS 4.3.1 (build 15T5567a) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-05-14T17:14:00Z. The update package is 146.11 MB. This is a delta (partial) update that requires 4.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 76c617a8c6e91a0910f6367586c90e4e17a44946.zip.