Without any doubt, Esperanto was very popular in Eastern Europe.
In a way it opened the world in the same way iternet does now.
I remember as a 14 year old kid, writing to penpals in Brazil, Iran, Poland, Czechoslovakia, The Soviet Union...
It made me connected to the world before everyone else.
I remember writing to this person in Iran in the early 80's, after USA and Iran played against eachother in football (soccer) game. I guess it must have been the World Championship.
Iran won that game, and shortly after, I wrote my friend - in Esperanto - to congratulate him with winning the game. In my next letter he thanked me, and wrote he was surprised that being a 'western' man, I dared to write about such delicate things. Esperanto does make people more open, or open people are more interested then others in Esperanto perhaps. Back to Eastern Europe...
In the 50's, 60's and 70's of the previous century, when the cold war isolated many of the eastern european world, Esperanto was a way to break through that wall and to keep contact with the rest of the world.
No wonder a lot of eastern european countries dedicated one or more stamps to this theme:
Hungary celebrating 70 years of Esperanto in 1957 - in remembrance of Zamenhof (twice airmail stamps)
Bulgaria hosting the 48th International Esperanto Congress in Sofia 1963
Yugoslavian Post celebrating 100 years of Esperanto - a very sober monochrome stamp, and on top a green star (verda stelo) symbolising Esperanto.
Other then the previous issues, the stamp itself has no Esperanto words (esperanta is the adjective of esperanto, but in this text it's used as a yugoslavian word).
Finally, a sheetlet of the German Democratic Republic (DDR) in 1987. On the stamp, the face of Zamenhof, and in the border of the sheet a bilingual text, Esperanto and German :
Esperanto im Dienste des Weltfriedens - Esperanto en servo de la mondpaco
Esperanto serving world peace
Für die Freundschaft zwischen den Völkern - Por amikeco inter la popoloj
For peace between people
Ohne Frieden keine Zukunft - Sen paco ne futuro
Without peace no future