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I have been unable to find a copyright notice anywhere for a short organ manuscript of a "Theme d'amour" from "Tristan et Yseult". It has been hitherto unpublished and the editors (self-publishing) of the first edition (whom I have spoken to) have stated that their edition is CC BY-SA. (https://imgur.com/a/4zvKWvb - join the Discord and scroll up if you need to see more)
Nor have I found a copyright notice on the first publication of two quarter-tone monodies for Ondes Martenot. This information has all been corroborated by a Messiaen scholar.

Update:
The same Messiaen scholar has failed to find a copyright notice on the first publication of a Piece for the Tombeau de Paul Dukas either. If the moderators have anything to say regarding the copyright status of these pieces, PLEASE reply.
(The original version published in the Revue Musicale in 1936 (similarly to the publication situation regarding Dechiffrage 1, NOT THE 1996 DURAND by the way)

Sorry go ahead and upload this.

Done.
Also, there's another piece that ended up somehow slipping through without a copyright notice: Sigle (1982) for flute.
Here's the information I got from the same Messiaen scholar:

Rolf Liebermann commissions the opera Saint François d'Assise from Messiaen.
In 1982, as the opera approaches completion, Messiaen decides to write two pieces as gifts for Liebermann: Fauvette des jardins (1980) and Sigle, a flute piece that emulates a blackbird.
The Sigle is recorded by Alain Marion for a TV program.
Anyway, now the Paris Conservatoire want a new sightreading piece. (They had previously commissioned Messiaen in the 1950s for Le merle noir)
Messiaen gives them the Sigle and they accept it.
It then stays unpublished until some time aroudn 2008, when a reproduction of the manuscript appears in the French edition of Hill and Simeone's book on Messiaen.

The book was published with a copyright notice but it was also the first publication of the piece, which is a separate work, but the piece itself went without a copyright notice.
A scan of the manuscript would be uploadable if we had a scan or copy of the original, but we don't have access to that, only copies of the book, and scanning from there would probably be violating the book's copyright. However, a new edition of the work should be fine.

Another piece that someone forgot to copyright in the US: Monody (1963) for organ.

The original publication (Schola-Cantorum) wasn't copyrighted in the US, but the later Leduc edition was copyrighted everywhere. HOWEVER, I DO NOT HAVE A COPY OF THE SCHOLA-CANTORUM EDITION.

What I have instead is a new edition and permission from the editors.

Do you have link evidence of the publication somewhere you can post here?

So I am afraid Skryabushka is correct, I must have missed this but we cannot have anything that was unpublished in the USA unless it was first published more than 95 years ago, otherwise it is life +70.