Germany Modern Censorship, What Is This Stamp?

Hello, I have two questions:
1 What is the story of this cover, was adressed to Berlin east from west Germany. It didn't found the recipiant, and was returned, but the return adress is Switzerland. So how those it works on those cases were the return adress is another country?
2. What is the stamp that got censored?

This is called Postkrieg. Certain countries for political reasons did not like some stamp designs from other countries. Mostly Eastern European countries. The unwanted stamps were covered by paint, there are a lot of different colors but black, grey and white are most frequent.

It seems to me that the "censored stamp" was the 1965 stamp on the 20th aniversary of the East German political migration to the West to escape from comunist rule. As it can be understood, East German authorities didn't like the stamp. It happened with a number of stamps issued by both sides during the "Cold War" (1947/90).

After the war, Germans from the west of Poland were forcibly expelled to Germany. West Germany commemorated the 10th anniversary of this "re-settlement" with a stamp in 1955. The Communist Polish government was not at all happy & mail bearing this stamp was returned. Polish propaganda film of the period shows these refugees boarding trains for Germany & much is made that they are "well fed & have regular seats", an obvious comparison with the plight of jews who were transported in cattle-wagons to their death during the war



Unfazed, West Germany issued a 20th anniv. stamp in 1965 with the same result

You are right , it is the 20th aniversary of the East German political migration to the West to escape from comunist rule stamp.
Thank you all