watchOS 4.3.1 (build 15T5545b) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2018-04-16T17:04:28Z. The update package is 362.29 MB. This is a delta (partial) update that requires 4.2.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: ef1686ec65a5fc916d4a62b55ba940a7931572b2.zip.