The City of Stettin
By the
Treaty of Versailles the navigation on the Oder became subject to international agreements, and following its articles 363 and 364
Czechoslovakia was entitled to lease in Stettin its own harbour bassin, then called (in German) Tschechoslowakische Zone im Hafen Stettin.
After WWII, Czechoslovakia argued for extension of the lease, given the new German-Polish border (specified at the Oder river) left the status of the city itself unclear. Following 1946, the city of Stettin (Czech: Štětín, Slovak: Štetín) was put under Czechoslovak administration. This caused protests by the Polish (both the Provisional Government of National Unity and the (irrelevant) London government in exile) and the relations between Poland and Czechoslovakia remained strained until, following the Czechoslovak February coup in 1948, a treaty of friendship between Czechoslovakia and Poland has been signed, mediated by the USSR. Poland recognized the sovereignty of Czechoslovakia over the city, while the port has been put under common Polish-Czechoslovak administration, making it de iure a condominium.
Czechoslovakia also asked for the control over the city of Königsberg, but Soviet Union
very firmly made it very clear that this is not a good idea (and indeed, the whole East Prussia has been divided between Poland and the USSR shortly afterwards). It is not clear if this was meant seriously, or it was only a joke or a mistake. In fact, this would be such a preposterous demand that many historians doubt the veracity of this claim.
Following the victory of Solidarność in Poland, the Velvet revolution in Czechoslovakia, the hitherto restrictive and secluded city was open for travel, and after the start of the German re-unification talks, the borders between Poland and Germany were quickly re-asserted as inviolable, but (west) Germany successfully renegotiated the status of Stettin with Czechoslovakia to as Czechoslovak-German condominium (for a hundred million marks paid to Czechoslovakia), while a separate agreement with Poland made the port itself a triple Czechoslovak-Polish-German administered area. (In practice his meant the citizens of Germany and Czechoslovakia had the right to work and reside in the city, people with permanent residentship in the city of longer than 5 years had the right to move and work in Germany; Germany, Poland and Czechoslovakia could freely use the port facilities. This became quite irrelevant after all the countries joined the EU.).
When Czechoslovakia split in 1993, the previous agreements were extended to cover both Czech Republic and Slovakia “jointly and severally”, the city thus became a Czech-Slovak-German condominium, and the port a Czech-Slovak-German-Polish one, an agreement that lasts till today.
In 1946, a special naval ensign for a (never to materialize) “Czechoslovak Baltic fleet” has been designed, using elements from the flag of the city, defaced with the Czechoslovak coat of arms.
This flag has been (unofficially) used throughout the city itself, alongside the Czechoslovak flag, while a similar version with the Polish coat of arms has been used by the Polish authorities in the port.
After 1993, there were several proposals for the new (and official) flag of the city, but including all the three (four) coats of arms was deemed impractical, and at the end, a simpler design incorporating the colours from respective national flags has been adopted, giving the flag its infamous garish look (and the less said about the numerous occasions when it is mistaken for an LGBT flag nowadays, the better).