In short: the 1923 date does not apply to unpublished works.

So far, we have only considered published works. What does published mean? A work is published when it is made accessible in some non-ephemeral form to the public at large with the consent of its author. Ephemeral forms of making the work accessible do not constitute publication. To quote the Berne Convention, §3.3:

The performance of a dramatic, dramatico-musical, cinematographic or musical work, the public recitation of a literary work, the communication by wire or the broadcasting of literary or artistic works, the exhibition of a work of art and the construction of a work of architecture shall not constitute publication.

As long as a work is not published, it is unpublished. (Also note that by publication, the work must be made accessible to the general public. Hanging a painting on one's bedroom wall does not constitute publication. It is unclear how to interpret the Berne Convention's phrasing on art exhibitions and building construction. How else could one "publish" such works if not by exhibiting or building them?)

Why is this important at all for Wikipedians? How could you come across an unpublished work?

One possibility is a photograph that was never published while it would have been copyright protected, turns up later in some collection, and then is published. Of course, it ceases to be an unpublished work at that moment. The rules of the U.S. copyright law are as follows:

• If the author and his death year are known, the copyright expires 70 years after his death, unless the eventual publication occurred between 1978 and 2002 (inclusive). In that case, the work is copyright protected at least until December 31, 2047.
• In all other cases (anonymous works, works made for hire, unknown author or death date unknown), the work is considered copyright protected until 120 years after its creation.

The Berne Convention leaves it to any signatory country to make its own rules regarding unpublished anonymous works (see §15(4)). Unpublished works by a known author, however, are subject to the same minimum protection (50 years p.m.a) as published works.