Max Silberberg owned Manet's La Sultane from 1928 to 1937, according to the Kunsthaus Zürich

The provenance of Manet's La Sultane, also known as Woman in Oriental Garb, has changed so often that it can be hard to keep up. The ownership by Holocaust victim Max Silberberg, which was challenged and undermined by speculation in previous iterations of the provenance published by the Bührle Foundation, has now been definitively confirmed by the Kunsthaus Zürich. 

Silberberg owned the painting continuously from 1928 to 1937.

The Kunsthaus Zürich publishes a page with "drawers" of documents and information from the collection.

https://www.kunsthaus.ch/en/buehrle-provenance-drawers/