Quite simple: The stamp of the library of Humboldt-Universität zu Berlin (Humboldt University Berlin).
The shape of the stamp makes it also quite visible on the page.
Fun fact: West Berlin had two different universities, but the Humboldt University in East Berlin never accepted the fact, that – since 1948 – was not the only university in town. So, up until the 1989 turnaround in Germany, all the books of the main library are stamped as “Universitätsbibliothek Berlin” (Library of the University of Berlin). Only afterwards they used the Name “Humboldt Universität zu Berlin”.