Allenstein, now is called Olsztyn, is a city in northeastern Poland. Allenstein became part of the German Empire in 1871 during the Prussian-led unification of Germany. From 18181910 the city was administered within the East Prussia Allenstein District, after which it became an independent city. In 1920 during the East Prussian plebiscite, Allenstein voted to remain in German East Prussia instead of becoming part of the Second Polish Republic. In order to advertise the plebiscite, special postage stamps were produced by overprinting German stamps and sold from 3 April. The stamps became invalid on 20 August. On 2 August 1945, the city was placed under Polish administration by the Soviets according to the Potsdam Agreement and officially renamed to Polish Olsztyn.