watchOS 4.3.1 (build 15T5555c) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2018-05-01T17:07:08Z. The update package is 171.25 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: 094919896c9a5671f4c6ed1380586f67a1ec32f9.zip.