watchOS 4.3.1 (build 15T5535b) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2018-04-02T17:13:31Z. The update package is 173.14 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: e18822d014612b3d9b6b43026171ce7f08bb7c5a.zip.