DI2DR was a 1965 Max Planck Atlantic Expedition

Organized by Max Planck Institutes for Aeronomie at the Institute for Ionospheric-Physics, Lindau, Harz (Germany).

DI2DR was a call sign used by Dipl. Phys. Hans U. Widdel, DJ2KS aboard the Research Vessel Meteor from August 8, to December 16, 1965 in the Atlantic Ocean.
Other members of the expedition were  Prof. Dr. Walter Dieminger, DL6DS, and Dr. Gerhard Rose, DL6XP.
Hans added a comment on the back of the card: "DI is a special call in DL, very rarely on ham bands!"
They used a Collins 75S3, Collins 32S3, Hygain 14AVQ Vertical

QSL Collection at DokuFunk, Vienna
Courtesy of OE1WHC
Info courtesy of W5KNE, OE1WHC