It is crucial to understand the distinction between the "Special Edition" (SSE) and the "Anniversary Edition" (AE). In technical terms:
The power of 1.6.640 lies in the "Address Library for SKSE Plugins." This library acts as a translator. While 1.6.640 is technically an "AE" version, it is the final version that major library authors verified before Bethesda changed the compiler flags. Newer versions (1.6.1130+) introduced code that confused the Address Library, leading to crashes on startup. skyrim special edition 16640 verified