On the 6-th of February, after 18 days of traveling I got this nice cover. It celebrates the 150-th anniversary of the first Polish stamp issue.
There is yet no information about the stamp on the official page, so I'll try to translate by myself. On the stamp you can see a fragment of the letter which was sent from Kalwaria to Warsaw on the 28-th of May 1860. On the top of the stamp you can see the addressee's building, e.g. The Bank of Warsaw in 1860.
Our post also issued a stamp, devoted to the 150-th anniversary of the Russian first stamp, so the cover will be a good edition of my collection.
Thank you very much, Andrzej!