The file metadata of the oldest copy on the gutenberg webserver says 2003 – and the document itself says Gutenberg created it in 2003 and published it in 2005 (whatever that means, maybe they were delaying ebook releases to ensure a steady stream)
Anyway this 2003 copy had their public domain boilerplate; it was described as a book in the public domain.
There are indeed a lot of websites about this, but none with any more information that Project Gutenberg so I’m guessing they all trace back to the Gutenberg release. Probably you’d have to find some physical information about it in an actual library to trace it further.
But I’m not like a professional book researcher or anything, that’s just my opinion!
(After sleeping on it it’s possible the book I was thinking of was written in the earliest 21st century).
There was an announcement on their mailing list here: https://lists.pglaf.org/archives/list/gmonthly@lists.pglaf.org/thread/MTHHI3TD7YXD3EHLKVBBA57KRBBWRI72/
Based off Wayback Machine poking around it looks like they were added sometime between September 20th to October 1st.