Hello Emily,
in case you have overseen my post some weeks ago:
Max Büttner: Trio for 3 harps (1929)
I own the only existing autograph manuscript of the work and have typeset the score. It now can be downloaded free of charge from my website in pdf-format:
http://www.tobias-broeker.de
There are also free scores for his Serenade for 2 harps and his work “Der Totentanz”, for reciter and harp.
Best,
Tobias