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Microsoft's latest skip ahead Windows 10 Insider build has two new hidden features that could suggest it's planning a move on mobile in its next major release.

Build 17650, the Redstone 5 preview released last week, has a new lightweight version of Windows 10 called Windows Lean.

Windows 10 Lean was revealed on Twitter by Windows enthusiast Lucan, who noted the heavily cut-down OS has no wallpaper and is missing apps like Registry Editor and Microsoft Management Console, as well as drivers for CD and DVD drives. Due to this trimming, the x64 install of Windows 10 Lean is about 2GB smaller than Windows 10 Pro.

See more: 20 pro tips to make Windows 10 work the way you want (free PDF)

Microsoft also refers to Windows 10 Lean as Windows 10 CloudE, which has a few quirks, including Edge not showing download prompts, meaning it's impossible to download files, and for some reason Office can't be installed.

It's not clear what Microsoft's plans are for Windows 10 Lean/CloudE and how it fits in with Windows 10 S, which at one point was also known as Windows 10 Cloud.

Windows 10 S, originally promoted as a new edition of Windows 10, is now known as 'S mode' and will be an option for all editions of Windows 10 next year, rather than just for Windows 10 Pro as it is today.

As ZDNet's Mary Jo Foley speculated in a chat with Lucan, Windows 10 Lean could merely be S mode for Enterprise or Home editions.

On the other hand, as Lucan noted, Windows 10 Lean doesn't have the same restrictions that Windows 10 S has.

See more: IT pro's guide to the Windows 10 Fall Creators Update (free PDF)

Another intriguing inclusion in this Redstone 5 preview is the appearance of phone-calling APIs that would support standard phone functionality such as dialing, video calling, and blocking withheld numbers.

As noted by Ars Technica, Windows currently has limited support for mobile modems to enable data connections, but actual calling required Windows Phone or Windows 10 Mobile.

The new phone-calling APIs could support speculation about Microsoft's long-rumored plans around Andromeda, a foldable Windows 10 mobile device that could serve as Microsoft's return to the mobile market.

Image: Lucan/Twitter/Microsoft

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