WinBeta posted a story that a new photo of Windows 8.1 build 9374 was shown on a popular Polish forum

Just a few hours ago, it was confirmed real due to a leak of the boot.wim and system32 contents. Since the build has no way of being run, all that can be looked at are the compilation dates and the file versions, but it is known that there is a new SKU dubbed "8.1 Pro Preview". Could Microsoft be on its next phase of Windows 8.1 development?

For those curious, the build string is 6.3.9374.0 (fbl_partner_out15.130329-2355) with a build date of 03/29/2013.

Windows Blue build 9364 was leaked on March 24th of this year and more screenshots of Windows Blue were subsequently leaked, showcasing an SKU of "Windows 8.1 Pro." Eventually, it was confirmed that Windows 8.1 would be the name for Windows Blue.

  Windows 8.1 build 9374 spotted in the wild