The version number of the first Windows 10 release was 1507. The number follows Microsoft's later naming convention: 15 identifies the year 2015, and 07 identifies July, the month of the original release.
Windows 10 began as the Threshold development branch and was publicly released on July 29, 2015. Its underlying build number was 10.0.10240. Microsoft initially referred to it simply as the original Windows 10 release; the 1507 label was applied retroactively when the company adopted year-and-month version names for subsequent feature updates.
This version introduced the returning Start menu, the Cortana assistant on desktop PCs, the Action Center, virtual desktops, and Microsoft Edge Legacy. The alternatives are later Windows 10 releases: 1607 was the Anniversary Update, 1703 was the Creators Update, and 1809 was the October 2018 Update. Thus, 1507 identifies the launch release rather than its technical build number.