Eastern Silesia was formerly the Austrian crownland Austrian Silesia, which was occupied by Czechoslovakia after World War I. Plans for a plebiscite fell through, and the area was divided between Czechoslovakia and Poland.The territory was entered by the Czech military in January 1919 after negotiations with Poland broke down. A final agreement on the border was reached in July 1920.
Eastern Silesia is remembered today by philatelists, since the plans for the plebiscite included the issuance of special postage stamps. These were stamps of Czechoslovakia and Poland, overprinted with various combinations of "SO" or "S. O." and "1920". They were in use from February to September 1920.The postal history of the territory is quite complicated with both overprinted and un-overprinted stamps of the contending countries (and Austria) in use at different post offices at different times.
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