Windows 10X build 19540

Windows 10X build 19540 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 19531 and 19561. Judging by the dates provided, it could have been compiled on or before 16 January 2020.

Source
Use the Microsoft Emulator - Microsoft Docs