On 26 April 1930, these airmail stamps were issued to commemorate the first South American flight of the LZ-127, Graf Zeppelin.
The stamps are the 1928 Zeppelin Airmails with an overprinted inscription of an eagle and “1. Südamerika-Fahrt” (First South American Flight).
Those with “X” suffixes have vertical watermarks, and those with “Y” suffixes have horizontal watermarks.
For similar overprints, see the later Polarfahrt and Chicagofahrt issues.