We reported on this (14342) milestone Windows 10 Mobile build here, a week ago, but a second tweak has been made, bringing battery life fixes, especially to older (non-native-Win-10) Lumias which have been upgraded by enthusiasts. These were apparently causing devices to experience excessive battery drain, though the Lumia 930 gets so hot in normal life it's tempting to say that it's hard to tell the difference....

From the Microsoft clarification:

We are releasing Build 14342.1004 today for Windows Insiders in the Fast ring. This build is exactly the same but with some additional fixes including a fix for the battery life issues we were seeing on some devices such as the Lumia 930, 929 and others which was also causing these devices to get hot.

See the original story for the full list of fixes for 14342.1001, but it was described by me as a milestone Redstone release because of the sheer number of new features and fixes. It hasn't been without issue on the editorial devices here, but hey, this is the 'fast' ring for Insiders, so what do you expect?! I do maintain at least one device on the 'slow' ring too, so that I can monitor progress.

As per all my recent charts and comments, note that Redstone builds are NOT available for the non-Win-10-approved phones (e.g. Lumia 920). no matter how many times you install the 'Windows Insider' utility and enthusiastically tap the 'Fast ring' entry.... The highest version of Windows 10, even unsupported, for older (mainly x20) handsets is 'Threshold' (the 10586.xxx branch). See my feature on all this if you're unsure where your phone will end up in Microsoft's roadmap.