The version number, 1.6.640, might seem like a random string of numbers to the uninitiated. However, it holds crucial information:
As of this writing, Bethesda has released newer versions (1.6.1130 and 1.6.1170). The version will not work on those newer executables. If you have updated past 1.6.640, you face a choice: sos 1.6.640
Most reputable modding guides (like those on LoversLab or the Skyrim Guild) will direct you to a repack that includes: The version number, 1