From Microsoft's notes on the production/Release Preview update*, build 494, available for both PCs and Mobile:
* the cycle seems to be one test release for Release Preview Insiders, then a fortnight's gap to the next, different version, but pushed out to both Insiders and production phones. So Insiders only really get to test every other version ahead of the general public, but then Windows 10 'Threshold 2' is supposed to be pretty stable by now, so there aren't any real surprises or gotchas.
This update includes quality improvements and security fixes. No new operating system features are being introduced in this update. Key changes include:
- Improved reliability of Windows Media Player, Internet Explorer 11, Windows Explorer, Miracast, and Windows kernel.
- Fixed issue with some people seeing a black screen instead of the lock screen when resuming from connected standby.
- Fixed issue with DirectAccess where the Disconnect button appears in some cases when the connection isn't fully established.
- Fixed issue where local user accounts weren't locked after a number of failed sign in attempts.
- Improved support for video playback in certain apps that didn't render video correctly after resuming from sleep.
- Fixed issue with Windows Phone losing user authentication certificates after upgrading to Windows 10 Mobile in some cases.
- Improved support of the backup settings of apps on Windows 10 Mobile.
- Fixed issue where Microsoft Silverlight based apps wouldn't install on Windows 10 Mobile after joining Azure Active Directory.
- Fixed additional issues in .NET, Windows Kernel, Windows Update, authentication, revised daylight saving time, support for PDF files, Bluetooth, Microsoft Edge, Internet Explorer 11, networking, and Wi-Fi connectivity.
- Security updates for Microsoft Edge, Internet Explorer 11, Kernel Mode Drivers, Windows Kernel, .NET Framework, Windows Secure Kernel Mode, and Microsoft Print Spooler.
All our production and RP Insider devices updated successfully.
Next, the Fast ring Redstone Insiders build was bumped up too, build 14388, with official notes:
In build news, today we are releasing Build 14388 for both PC and Mobile to Windows Insiders in the Fast ring. This build has 44 new fixes in it including a bunch of fixes to improvement reliability and battery life and improvements to accessibility. A few of the most notable fixes are listed below.
- We have updated the Store to version 11606.1001.39 with bug fixes.
- We fixed the issue causing the keyboard to not show up consistently for InPrivate mode on Microsoft Edge.
- We fixed an issue that could result in an app getting into a state after being suspended where it would be unable to launch until rebooting the device.
As usual, there are some known issues:
- Voice Recorder does not show up consistently as an option for Call Recording. We heard your feedback and plan to release a Voice Recorder app update via the Store soon that has the fix for this issue.
- Wallet users are prompted for a PIN twice when using tap to pay from a locked phone. You can enter their PIN twice and tap as usual once the phone is unlocked.
- REMINDER: We have changed the backup format for Windows 10 Mobile devices to reduce the size of the backup stored in OneDrive. As a result, if you do a backup on a device running the latest Windows 10 Mobile Insider Preview builds and move back to the released version of Windows 10 Mobile (Build 10586) and restore from your backup – your Start screen layout won’t restore and remain the default Start layout. Your previous backup also gets overwritten. If you need to go back to Build 10586 temporarily, once you are on Build 10586 you should disable backup so it doesn’t overwrite the good backup from Windows 10 Mobile Insider Preview builds.
I currently only have one phone on the Fast ring, the Lumia 950 XL, but this updated successfully - pop-up media controls continue to be an issue for me, but at least it's good to be reading PDFs again!
Comments welcome on either of these new branch versions of Windows 10 Mobile.