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Here an ethnic map of Lithuania and Belarus from the Russian Atlas Narodov Mira (published in 1964):
https://s1.postimg.org/47zbdhp3t9/p0016.jpg
You can see the so called "Pas Polski" ("Polish Strip") from the border of Poland all the way to Latvia:
And remember, that over half of Polish population from this area was deported to Poland after WW2.
Despite that huge deportation, ethnic Poles are still the majority of inhabitants in large areas there.
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