You can also find this version in the "Old Files" section of the Skyrim Script Extender (SKSE64) Nexus page. How to Install
This means your SkyrimSE.exe updated via Steam. You will need to either update SKSE to the newest version or use a downgrade patcher to return to 1.6.640. skse 16640 link
To run mods on Skyrim version 1.6.640, you specifically need . Using a newer version (like 2.2.6+) or an older one will result in a version mismatch error when you try to launch the game. You can also find this version in the
Look for the "Current Anniversary Edition build 2.2.3 (game version 1.6.640)" link. skse 16640 link