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
Use the Microsoft Emulator - Microsoft Docs