The story begins with a Windows 7 Ultimate OEM DVD ISO from Lenovo leaking to a Chinese forum. The boot.wim file was then used to retrieve the OEM-SLP product key and OEM certificate for Windows 7 ...

The key is for use with Windows 7 Ultimate RTM product that is meant to be pre-installed by the OEM on new PCs to be shipped later this year.

In spite of the official end of the free Windows 10 update offer on July 29, it seems any valid Windows 7/8.x retail product key still installs Windows 10 for now.