So for Stalinists to catch up with Banderists - you need to extrapolate very high death rates for Poles deported to Central Asia and Siberia.
Given the number of these Poles still alive in the 1970s and 1980s, and how many later repatriated - it stretches belief that all of them died a cold and famished death in the steppes of Kazakhstan.
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What we have left to tune up the numbers is the "Polish Operation" of the NKVD.
Problem there is that a large part of those killed were Soviet Poles, not Polish Poles.