BRIEF [O.A.] Hackenbrock, Leon Author: Hackenbrock, Leon Recipient: Haeckel, Ernst Dating: 17.01.1905
BRIEF [O.A.] Hackenbrock, Leon Author: Hackenbrock, Leon Recipient: Haeckel, Ernst Dating: 27.01.1905
BRIEF [O.A.] Hackenbrock, Leon Author: Hackenbrock, Leon Recipient: Haeckel, Ernst Dating: 13.12.1905