It's been over a month since the last Windows 10 Mobile preview build, 10166. Now, a newly leaked screenshot of an alleged new version shows that the next build could take a big leap, with the number 10512.
The image was posted by the Chinese-based site ITHome, which has been a reliable source of previous Windows 10 Mobile screenshot leaks.
Previously, Windows Insider lead Gabriel Aul said that Microsoft had a version of Windows 10 Mobile with the build number 10240, but declined to release it to the public due to a "blocking bug" that was discovered. Later on, Aul noted that they were creating a new branch of the OS for Insiders, which is likely why this is part of the 105xx series.