These magnets depict three 1930 U.S. stamps that commemorated the Graf Zeppelin airship that completed its first Europe-Pan American round trip flight in May of that year. The stamps were sold to the general public from April 19 - June 7, 1930 and for an additional 3 weeks to stamp collectors.
Zeppelins were rigid dirigibles pioneered by a German count, Ferdinand von Zeppelin. The design was so successful that the name Zeppelin came to be applied to all rigid dirigibles. Passenger service was interrupted by WWI while Zeppelins were put to use as bombers and scouts. Passenger and cargo service resumed after the war and flourished in the mid 1930s but Germany's WWII war propaganda and preparation ended the Zeppelin line by 1940.
Please note that these are flat, flexible magnetic pictures of Zeppelin stamps and are not actual stamps.