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Map of Silesian Voivodeship showing the Polish-Czech border in 1938-1939 (you can zoom in):
https://polona.pl/item/wojewodztwo-s...#info:metadata
Here you can see the Olza river, Polish-inhabited area to the west of it is called Zaolzie in Polish:
Zaolzie area annexed by Poland in 1938, had 200,000 ethnic Poles (almost 90% of population).
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