This article discusses software internally used by Apple.
Acquiring a copy without Apple's consent is illegal and may result in being scammed.
The following build is a factory firmware built around an internal version of iOS 7.0. Its filesystem is nicely sized; around 3.1 gigabytes.
This firmware has the kernel version
Darwin Kernel Version 14.0.0: Tue Aug 13 21:48:05 PDT 2013; root:xnu-2423.1.73~3/DEVELOPMENT_ARM64_S5L8960X.
Sadly, the dump of this build is in ZIP format, meaning that file permissions and symlinks are lost.
/AppleInternal/Restore/FactoryTests/ you will find codenames for unreleased devices including
N61 codename for the iPhone 6 is also present, meaning Apple likely had prototypes of the iPhone 6 at the time this build was built.
After booting up, SwitchBoard.app lets you choose from a decent amount of apps. They are: