Microsoft is now testing a new build of Windows 10 Mobile Version 1511 internally. Recently, Microsoft pushed build 10586.338 to Windows Insiders in the Release Preview Ring. However, the company has started testing a new build: 10586.420. Just like most of the previous 10586.xxx updates, this build does not include any new features. Of course, the build should include a few bug fixes and improvements which are always appreciated.