On August 10, Microsoft released two new Windows 10 test builds: A "Redstone 5" test build to the Fast Ring and a "19H1" build to the Skip Ahead Ring.
(Just a reminder: 19H1 is the replacement codename for the build that many had assumed would be called "Redstone 6." It's the feature update to Windows 10 that will follow Redstone 5 and will be released around April 2019, as far as we know.)
The Fast Ring test build, No. 17735, includes no new features, but quite a few fixes. It also includes many known issues, which are listed in the blog post. Microsoft is expected to roll out Redstone 5 to mainstream users starting around October 2018.
The Microsoft "Your Phone" app now works with Build 18214, as it alreaedy does for Redstone 5 testers. With the current build on Android, testers can get immediate access to their most recent Android photos on their PCs, so they can copy, edit or ink on those photos. On iPhone, the YourPhone app only allows users to pick up on their PC where they left off in their browsers on their phones.
It's not unusual, at this point in the Windows development process, that the Fast Ring and Skip Ahead Ring builds are basically the same. (The inclusion of the basically completed dark-themed File Explorer is one of the features that is in Redstone 5 now, but not yet in the 19H1 build.)
Microsoft will likely not begin adding any major new features to Skip Ahead builds for a while -- most likely some time after Redstone 5 is deemed "baked" and closer to roll out to the mainstream.