watchOS 4.3 (build 15T5199f) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-03-07T02:04:19Z. The update package is 324.41 MB. This is a delta (partial) update that requires 4.2.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: a470b3c2b9af06fc10040cf731750f6dd24d2cc0.zip.