Running Grepping/Strings on sources/Setup.exe: This seems to show the most accurate version number, including an internal build number. I have yet to install this ISO, so I am not sure if the 151029 number is indicative of it being a 10.0.10586.29 build number. The command below uses Sysinternals' strings program:
> strings setup.exe | findstr -i 10\.
DLL Method: Once you've extracted the files from your Windows 10 .ISO file, find at any DLL file in the Sources folder. Right-click it and choose Properties. Then click the Details tab. Under Product Version or File Version, it will show the build number:
Sources/Ws.dat: This file is in the sources folder. This file is in the inf format. Open it in notepad and it shows the build in the ClientVersion field. Note that in the 10240 build this file was empty, but going forward it might be used in the future.
Sources/Idwbinfo.txt: This file is in the sources folder. Open it in notepad and it will show the major release. th1_release is for 10240 and th2_release is 10586:
BuildBranch=th2_releaseSources/schema.dat: This file is in the sources folder. It's a binary file, but you can still open it in notepad. Search it for the string version and you'll see the full build number
Sources/sxs/microsoft-windows-netfx3-ondemand-package.cab: This file is in the sources/sxs folder. Open the file with 7-zip, and the file names will show the Windows build number:
Here's my copy of the Media Creation Tool, version 10586, which as of this post, will still download a build 10586 ISO. If you have the original July 10240 Media Creation Tool, it will download the RTM build 10240 ISO.