Yesterday Apple released beta 1 of watchOS 4.2.2 to registered developers. The build number is 15S5530d.
The release comes at the same time as iOS 11.2.5 beta 1. The watchOS 4.2 SDK provides support for developing watchOS apps for watchOS 4.2.2. The SDK comes bundled with Xcode 9.2 available from the Mac App Store.