Cover from Poland

On the 23-rd of March after a month of traveling I got this cover from Poland. This time it was devoted to the lost works of art. It's well-known that during many wars some works of art could be stolen or lost. And that also happened during the WWII. On the souvenir sheet you can see the pictures of A. Durer, P. P. Rubens, Rembrandt which were  lost from Polish museums and libraries during WWII. Of course, it's not all of them, but still it's a way to attract attention to this important problems for national heritage of every country!