Short answers, pulled from the story.
The Polish Corridor was a strip of land connecting Poland to the Baltic Sea between 1919 and 1939. It existed as part of the Second Polish Republic following the Treaty of Versailles signed on the 28th of June 1919.
Polish politicians first used the word corridor in political discourse during the early 1920s according to German historian Hartmut Boockmann. The English language adopted the term by the 18th of March 1919 when The New York Times published a sketch of the proposed territory.
By 1910 the Prussian census showed 528,000 Poles including Kashubians compared to 385,000 Germans living in the region. Richard Blanke noted that 421,029 Germans represented 42.5% of the total population while ethnic Poles made up between 36% and 43% depending on the source.
The draft Treaty of Versailles was signed on the 28th of June 1919 and took effect on the 20th of January 1920. Articles 27 and 28 defined the shape of the Polish Corridor while articles 89 to 93 covered transit rights and citizenship issues establishing Poland's access from 70% of dissolved West Prussia.
Adolf Hitler demanded the return of Danzig and a plebiscite for the corridor starting on the 29th of August 1939 when Ribbentrop handed British Ambassador Sir Nevile Henderson a list of terms. He proposed an exchange of minority populations where all Germans born but not living there would vote while Poles born after 1919 would have no vote.