Microsoft today released a new Windows 10 Mobile build to the Fast ring, bringing the build number to 15205. It comes alongside PC build 16179, but unlike that one, it's not exactly from the Redstone 3 branch.
However, it's not exactly Redstone 2 either. Build 15205 is something called 'feature2', which we still don't know a whole lot about. Some have speculated that it's a Windows Phone 7.8-type update that spells doom for Windows 10 Mobile, while there have been other rumors that phones will just have to wait until the Creators Update rolls out next week to be brought up to speed with Redstone 3.
Unfortunately, like build 15204 which arrived five days ago, it's only available to 13 handsets. These basically include devices that shipped with Windows 10 Mobile, along with the Microsoft Lumia 640 and 640 XL. It also mirrors the list of devices that will receive the Creators Update through the Production ring (although unsupported devices can still get it via Release Preview).
Naturally, this build contains no new features for the OS, and whatever the case may be with feature2, Microsoft is continuing to roll out Insider Preview builds, so we'll find out more as time progresses.