Sometimes used for split binaries of large open-source projects.

Knowing the source would help me give you a much more detailed breakdown of what is inside "1266."

Ensure you have every numbered segment associated with the "1266" set.

Without more context, it is difficult to provide a "review" of the actual data inside. However, these specific numbered files often appear in the following communities:

Use a tool like 7-Zip or WinRAR.