Microsoft is actively working to test a new build of Windows 10. Gabriel Aul teased on Twitter how 10550 is currently on Microsoft's internal ring, but could make it out to testers soon. With Wave 2 drawing closer, one would assume Microsoft would drop a new build soon enough to pack as many bug fixes and improvements in before release.
@callum90ish 10550 is in Microsoft ring now. Only a few builds newer than last flight but may go out to WIP Fast. We'll see.— Gabriel Aul (@GabeAul) September 24, 2015
Build 10547 is currently in the fast ring for participating testers. We'll keep an eye out to see if build 10550 drops before the month is out.