A busy 'Good Friday' for Microsoft, pushing out builds right, left and centre, mere minutes before everyone decamps for Easter. Of note for mobile is that Microsoft is only implementing fixes on top of the Redstone 2 codebase - for now. Does this mean that 'Redstone 3' won't explicitly happen and that Microsoft will bring forward 'Windows on ARM' for all? I guess we'll find out in time. Let the comments rage...!

From the Microsoft announcement:

Today we are excited to be releasing Windows 10 Insider Preview Build 16176 for PC to Windows Insiders in the Fast ring. We’re continuing work to refine OneCore with some code refactoring so that teams can start checking in new code. So, you still won’t see any noticeable changes or new features in new builds just yet.

We are also releasing Windows 10 Mobile Insider Preview Build 15204 to Insiders in the Fast ring. As we release new builds from our Development Branch for PC, we will also be doing the same for Windows 10 Mobile just like we have been in the past. However, Windows Insiders will likely notice some minor differences. The biggest difference being that the build number and branch won’t match the builds we will be releasing for PC. This is a result of more work we’re doing to converge code into OneCore – the heart of Windows across PC, tablet, phone, IoT, HoloLens, Xbox and more as we continue to develop new improvements for Windows 10 Mobile and our enterprise customers.

Changes, improvements, and fixes for Mobile

• We have added a new privacy page to the Windows 10 Mobile OOBE experience that allows you to quickly and effectively make common privacy changes while setting up the device. You can read more about our Windows 10 privacy journey here.
• We fixed the issue where the keyboard would sometimes not appear when a text input field is selected in Microsoft Edge.

Known issues for Mobile

• FYI: If you receive error 800b0109 while attempting to download this build, navigate to Settings > Update & security > Windows Insider Program and check for a restart message – please restart your device from there and then check for updates again and should be able to download the build as expected. 
• For Insiders who have upgraded from a prior 150xx build to this build, the “Add Bluetooth or other devices”� Settings page and the Connect UX page may fail to open.
• Some users are reporting that pages are constantly reloading or refreshing, especially while they are in the middle of scrolling them in Microsoft Edge. We’re investigating.
• There is an issue with Microsoft Edge where you might get into a bad state after opening a new Microsoft Edge windows and screen off with the JIT process suspended.
• Continuum will stop working when HP Elite X3 case is closed.
• Continuum hangs or renders incorrectly after disconnecting on devices like the Lumia 950.
• The device screen might stay black when disconnecting from a Continuum dock after screen has timed out normally.

So we're now beyond the build of the Creators Update and well into new territory - RS2+, RS3? Who knows, but at least critical fixes and improvements are being made to Windows 10 Mobile 'Fast' ring in parallel to the higher-numbered builds for PC.

Anyone with an existing phone that's officially deemed Windows 10 Mobile CU-compatible by Microsoft can join in the fun. In each case:

1. Upgrade to Windows 10 Mobile (if appropriate) using the Upgrade Advisor.
2. After at least one update cycle you should then be on the Anniversary Update (a.k.a. Redstone). Head into Settings/Update & Security/Windows Insider Programme
3. Join the Insiders programme and choose the 'Fast' level in the pick list.
4. Within a few hours your phone should be seeing the build mentioned above and you'll be running the Creators Update and beyond... 

In addition to the above, Microsoft included this information:

Starting with the Windows 10 Creators Update, these are the Windows 10 Mobile devices we will officially support in the Windows Insider Program going forward:

• HP Elite x3
• Microsoft Lumia 550
• Microsoft Lumia 640/640XL
• Microsoft Lumia 650
• Microsoft Lumia 950/950 XL
• Alcatel IDOL 4S
• Alcatel OneTouch Fierce XL
• SoftBank 503LV
• VAIO Phone Biz
• MouseComputer MADOSMA Q601
• Trinity NuAns NEO

Devices not on this list will not officially receive the Windows 10 Creators Update nor will they receive any future builds from our Development Branch that we release as part of the Windows Insider Program. However, Windows Insiders who have devices not on this list can still keep these devices on the Windows 10 Creators Update at their own risk knowing that it’s unsupported.

We recognize that many Insiders will be disappointed to see their device is no longer supported. We looked at feedback from our Windows Insiders and realized that we were not providing the best possible experience for our customers on many older devices. That helped us determine which devices we support for the Windows 10 Creators Update. We are continually listening to your feedback to provide the best experience for ALL of our customers.

This is, of course, poppycock. Users will get a much better 'experience' with Windows 10 Mobile on the Lumia 930 and 1520 than on the low end OneTouch or the Lumia 640. So this, again, is just Microsoft using the age of a device as the cut-off point. This is a common trait in the mobile industry but that doesn't make it right - in this case, in my opinion.