Windows 10 build 10240 (th1) is the official RTM build of the original release of Windows 10 originally released to Windows Insiders in the Fast and Slow rings on 15 July 2015 and later generally released to the public on 29 July 2015. It is the first build to become available for consumers to install and was also available as a free upgrade for Windows 7, 8, and 8.1 users until 29 July 2016.
Build 10240.16384 is the last RTM build with the build number divisible by 16 and the delta number bumped to 16384. This is a remnant of the older 20-bit revision numbering scheme, which used the most significant bits in the delta number to encode extra information. The 4 least significant bits of the build number were reserved for the Service Pack number, although Microsoft hasn't produced a Service Pack since the release of Windows 8. 2b1af7f3a8