Microsoft may soon release Windows 10 19H2 to public and it may usher in 20H1 build pushed to Fast and Slow Rings. This may also make the Skip Ahead get first Windows 10 21H1 preview builds, the next major feature update after 20H1.

Now, it seems that Windows 10 Build 19504 may be the first 21H1 preview build. Leaked screenshots of download related to the Build 19504 has leaked on Twitter (ALumia).

Windows 10 20H2 update will be a minor update similar to 19H2 and the build number 19504 is a big jump over the 20H1 build number which are numbered as 190XX. So one can safely assume that Build 19504 belongs to the Windows 10 21H1 update.