Eastern Silesia was formerly the Austrian crownland Austrian Silesia, which was occupied by Czechoslovakia after World War I. The territory had an area of 5,146 km² with a mixed German, Polish and Czech population of more than 680,000. The capital was Opava. Both Czechs and Poles lived in parts of Eastern Silesia, and they were unable to reach agreement on where the border should be drawn after the war.
The Allies were asked to arbitrate, but they were unable to arrive at a solution acceptable to all parties, so on September 27th, 1919, a plebiscite was called. The plebiscite was to be held under the auspices of the Allied Powers rather than the League of Nations and its Inter-Allied Control Commission (IACC). Eastern Silesia is unique in that the proposed plebiscite was not under IACC auspices, and, while plebiscite stamps for the region were issued by both the Czechoslovakian and Polish governments, the plebiscite was never held. The Soviet invasion of Poland in 1919-20 made the Poles more amenable to an arbitrated solution. The plebiscite was canceled, and the border was set by the Allied Conference of Ambassadors and accepted by both parties in July 1920.
Czechoslovakia and Poland, overprinted stamps with various combinations of "SO" or "S. O." (Fr. Silesie Orientale), and "1920" for the plebiscite. They were in use from February to September of 1920.
In 1938, at Nazi Germany's suggestion, Poland seized the disputed territory while Czechoslovakia was being dismembered after the Munich Conference. Germany annexed the territory during WWII, but the previous borders were reestablished after the war.
Polish stamps issued for Eastern Silesia:
Polish puppet-state Central Lithuania also issued a set of stamps commemorating the plebiscite:
These stamps were not used in Eastern Silesia.