BRIEF [O.A.] Kühnel-Engelsberg, Rudolf Author: Kühnel-Engelsberg, Rudolf Recipient: Haeckel, Ernst Dating: 18.08.1912
POSTKARTE Kühnel-Engelsberg, Rudolf Author: Kühnel-Engelsberg, Rudolf Recipient: Haeckel, Ernst Dating: 21.08.1912
BRIEF [O.A.] Kühnel-Engelsberg, Rudolf Author: Kühnel-Engelsberg, Rudolf Recipient: Haeckel, Ernst Dating: 15.08.1912