A New Windows Threshold Build 10.0.10586.338 has been sighted which Microsoft is internally Testing for Windows 10 Mobile and Windows 10. Being compiled on 17th May 2016, this could be the another Cumulative update  taking the place of the last released build 10586.318 presuming might not include any new features except some under the hood fixes and improvements for the OS.

Microsoft is likely to release Windows 10 Mobile Build 10586.338 and Windows 10 Build 10586.338 very soon.