Microsoft has released Windows 10 Mobile build 14267.1002 for anyone owning a Lumia 950, 950 XL, 550 or Xiaomi Mi4 and who's signed up to the Insider programme. This is the first release for mobile on the new 'Redstone' branch of the OS, putting Windows 10 Mobile on par with Windows 10 for PCs and tablets. The limited device release is probably for ease of testing - it would break too much on the older devices at this stage. Plus the newer chipsets will run the code faster and provide a better experience - it's not known whether all of this code will ever get to the full range of older Windows Phone 8.1 devices (1020, 920, etc.)

From the Microsoft announcement post:

Today, we are releasing Windows 10 Mobile Insider Preview Build 14267 to Windows Insiders in the Fast ring. While we have been releasing preview builds for PC from our development branch for a while now, this is our first new preview build for Windows 10 Mobile from our Development Branch this year. As I mentioned a couple of weeks ago, initially we’ll make new builds from our Development Branch available only to devices that shipped with Windows 10 Mobile. Currently this includes the Lumia 950, 950 XL, 650, 550, and Xiaomi Mi4. We plan to expand from there as we release Windows 10 Mobile to other devices.

Just to quickly recap the changes we’ve made to rings for Insiders in 2016: (I included this in yesterday’s PC build, but including here for Mobile for completeness):

1. We introduced the new Release Preview ring last week which is for Windows Insiders who want to stay on the Current Branch (currently builds based off of Build 10586), but continue to receive early access to updates, application updates, and driver updates. This is a great option for people who want to get updates early but not go all the way to installing full preview builds. Because the Release Preview ring offers updates to the Current Branch, it is only visible to devices on the Current Branch (Build 10586). If you are on a newer build, flights to the Release Preview ring won’t be applicable to that device. The easiest way to go between the Development Branch to the Current Branch on your phone is to use the Windows Device Recovery Tool for phone to reinstall Windows 10 Mobile.
2. We’ve adjusted the criteria we use to release new builds to Windows Insiders in the Fast ring to get builds out more quickly – this means these builds may include more bugs and other issues that could be slightly more painful to live with. This applies to mobile builds just like it does for PC builds. So if you’d like to change your ring – open the Windows Insider app > tap on “Get preview builds” > Choose the Slow ring.

Here’s what’s new

• Search for music easier in Cortana: We have made it easier to invoke music search in Cortana by adding a music search icon to the top right of Cortana. You can now open Cortana and tap on the music search icon to have Cortana listen and search for the song that’s playing.
• Word Flow now works in the address bar in Microsoft Edge on your phone:For fans of Word Flow, we heard your feedback, and we have re-enabled the ability to use shape writing in the address bar in Microsoft Edge on your phone!
• Open InPrivate tabs more easily in Microsoft Edge on your phone: We take the need for private browsing seriously, and InPrivate has been a feature of Microsoft Edge in Windows 10 Mobile from day one. However, we received feedback from Windows Insiders that the InPrivate feature was either hard to find or missing on their phones. As a result, we’ve moved the button to create a new InPrivate tab to appear right next to the new tab button. The next time you need a private tab in Microsoft Edge, you’ll find it right in your tabs view. Try it out on your phone and let us know what you think.

• Improved Download Prompts in Microsoft Edge: Microsoft Edge will now show a prompt when you start a download, allowing you to cancel unwanted downloads.
• Messaging + Skype improvements: You can now attach photos to your Skype messages by tapping on the paperclip icon at the lower left as you’re composing a new Skype message to a contact. Additionally, you can also invoke the Camera app to take a photo to attach to your Skype message too or send along your location.

Known issues

• Visual Voicemail notification and delivery is not working in this build; causing devices to not receive any new visual voicemails. You can manually check your voicemail by calling your voicemail to listen to any new messages. You can call your voicemail within the Phone app by clicking “…” > Settings > “Change more settings for Phone” > “Call voicemail”; this will also leave the Voicemail call in your history to make checking voicemail easier.

All well and good, I'm installing this on our Lumia 950 XL and will report back in due course. Note that this is a big update, just downloading it took many minutes on Wi-fi, I'm assuming something in the 1-2GB region. And it will take an eternity to install, of course, so allow for this on your 950 or 950 XL. The huge version number jump reflects the switch to a whole new branch of the OS, bringing mobile into step with the current Insiders desktop releases.

This can be considered the first real beta of the OS on the Redstone branch and so make allowances for increased battery drain and other lack of optimisations. My tests and features on improving battery life on the current branch (10586) of Windows 10 Mobile are ongoing!