watchOS 4.3 (build 15T5212a) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-03-16T20:09:18Z. The update package is 336.49 MB. This is a delta (partial) update that requires 4.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 6c8b167437675bea3bcc28699f66785b18c49514.zip.