This series was issued on 16 March 1936 to commemorate the voyage of LZ129, Hindenburg, to America.
Those with “X” suffixes feature a normal swastika watermark — when viewed from the rear, the legs of the swastika point in a counterclockwise direction. The “Y” suffixes feature the rarer reversed swastika watermark, with the legs of the swastika pointed in a clockwise direction when viewed from the rear.
Mint issues which have not had the gum removed tend to yellow due to the acidic content of the gum. This is the same issue that plagues the 1935 OSTROPA sheet.