watchOS 4.3.1 (build 15T5565a) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2018-05-07T17:23:31Z. The update package is 180.7 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: f528991d89c3f39dc535983649da509a8bc16150.zip.