Windows 10X build 19531

Windows 10X build 19531 is an unleaked build of Windows 10X. This build is mentioned in a screenshot on Microsoft Docs in the article about the Microsoft Emulator, along with builds 19540 and 19561. Judging by the dates provided, it could have been compiled on or before 6 December 2019.

Source
https://web.archive.org/web/20210227154455/https://docs.microsoft.com/en-us/dual-screen/windows/use-emulator Use the Microsoft Emulator - Microsoft Docs]