As you may already know, Windows 10 Build 16251 was recently released to Slow Ring Insiders. Microsoft today released official ISO images of Windows 10 build 16251 for those who want to install this build from scratch.
Today, Microsoft released a new build of Windows 10 to Fast Ring Insiders - Build 16257. Right after that, they made a gift for Slow Ring participants of the Windows Insider program. Windows 10 build 16251 is available for everyone who has configured their OS to receive updates from the Slow ring.