BRIEF [O.A.] Sauberzweig, von Author: Sauberzweig, von Recipient: Haeckel, Ernst Corporation: Gouverneur der Stadt Dating: 28.01.1916
POSTKARTE Hinderdael, Irmgard van Author: Hinderdael, Irmgard van Recipient: Haeckel, Ernst Dating: 31.12.1913
POSTKARTE Hinderdael, Irmgard van Author: Hinderdael, Irmgard van Recipient: Haeckel, Ernst Dating: 09.05.1918
BRIEF OHNE UMSCHLAG Richter, Wilhelm Author: Richter, Wilhelm Recipient: Haeckel, Ernst Dating: nach 1915
BRIEF [O.A.] Penninckx, Henry Author: Penninckx, Henry Recipient: Haeckel, Ernst Corporation: Jeunesse Progressiste Dating: 03.08.1910