watchOS 4.3.1 (build 15T5565a) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-05-07T17:23:15Z. The update package is 326.07 MB. This is a delta (partial) update that requires 4.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 271ac3ab0aa0fc6ee36843e37849c05dfd74c3ef.zip.