It looks like post-Versailles Europe (1920-39), when Danzig was a free city. Oppeln is a little more interesting since it held a plebiscite in 1921 as to whether or not become part of Poland or Germany. It ended up going with Germany, but there had already been a conflict going on in Upper Silesia between the German and Polish populations, culminating in the Battle of Annaberg in May 1921. To my thinking, this would place the map pretty shortly after Versailles, when the situation in Oppeln was still fluid.