Muckrock, a group specializing in FOIA requests and analysis, has released an article with attached documents concerning SM X (Security Matter X.) In this particular case, it is largely connected to the Maury Island Incident.

https://www.muckrock.com/news/archiv...onnection-bet/

So, SM X files are now confirmed to exist and to be the real deal. I, for one, will be watching with interest as more digging into this occurs.