Finding aid http://hdl.library.upenn.edu/1017/d/pacscl/UPENN_RBML_MsColl575
In German and French.
Rudolf Mengelberg, musicologist and composer, was the director of the Concertgebouw Orchestra in Amsterdam; he was a nephew of Willem Mengelberg, who was a friend of Alma Mahler. Laqueur was apparently an acquaintance of Rudolf Mengelberg and of Alma Mahler.
1 item is a letter concerning Rudolf Mengelberg's assistance in the promotion of Alma Mahler's memoir on Gustav Mahler (Erinnerungen und Briefe), in conjuction with the publisher Allert de Lange.