Six days after landing on the Fast Ring, Microsoft officially released Windows 10 Build 18912 ISO image, which is the first official ISO image of the 20H1 development branch. Interestingly, Microsoft did not remove the Build 18343 image released in March this year after the new image was released.
In fact, this is a mirror image of two different groups labeled Fast Ring and Slow Ring. Build 18343 is not actually the latest version of the Slow channel. The RTM version number of Windows 10 May 2019 (Version 1903) is Build 18362.
The Fast channel has already started testing the 20H1 branch version, and this feature update will go live next spring. Microsoft also plans to launch the 19H2 update later this year. I believe it should be concentrated on the Slow channel, so the 19H2 development branch will not be updated much.
At present, the ISO image of Windows 10 Build 18912 has three SKUs for users to download, Windows 10 Insider Preview, Windows 10 Insider Preview Enterprise and Windows 10 Insider Preview Home China. Interested users can go here to download.