BRIEF [O.A.] Ferber, M... R... Author: Ferber, M... R... Recipient: Haeckel, Ernst Dating: 23.06.1868
BRIEF [O.A.] Dessen, M... Author: Dessen, M... Recipient: Haeckel, Ernst Corporation: Russische Zigaretten-Fabrik "Kaukasisch" Dating: 29.03.1910
BRIEF OHNE UMSCHLAG Engel, Richard Author: Engel, Richard Recipient: Haeckel, Ernst Dating: 12.04.1918
