I have a copy of the 3 cello duets Op.22 by F A Kummer (died 1879) edited by J Klengel (died 1933). This was published by IMC (plate 768) and was first issued in 1946. The typesetting is in the IMC house style.
As I understand it, if I can show this is a reprint of an earlier edition published before 1923, then it will be public domain and I can upload it. There are many cases like this on IMSLP (e.g. see Cello Sonata in E major (Valentini, Giuseppe)). The fact that the editor (Klengel) died over ten years before this IMC edition suggests this may be the case.
The Kummer duets were originally published by Breitkopf und Härtel, who were also Klengel’s publisher. On the back of the first violin part in IMSLP of the Anton Rubenstein Op.17 No1 string quartet is a Breitkopf und Härtel advertisement. Under cello duets it has this entry;
3152, Kummer, Drei Duos Op.20 (Klengel).
Op.20 by Kummer is not a set of cello duets. Kummer only wrote 2 sets of 3 cello duets – Op.22 and Op.165, so I believe this is a misprint and should say Op.22. This plate number (V.A.3152) would give a date of about 1909.
Despite an extensive search, I cannot get any further.
So I have 2 questions that I would appreciate some help with;
1. Can anyone confirm that the cello duets referred to in the advertisement are indeed Op.22?
2. If no one can help, then is the information above sufficient to justify uploading it as being public domain?