This fixes the grammar.
"Souvenir Sheets issued to commemorate the 250th anniversary of the death of Ivan Mazepa
issued by the Organization for the Rebirth of Ukraine, Chicago, USA, 1959"
I am not sure what is meant by a "souvenir sheet". You could use the word "document(s)" which would be very general. A "souvenir" is usually something tourists buy to remember their trip, so we don't want that in this case. A "pamphlet" would be a small number of pages in a little booklet. Since you are explaining why the document or pamphlet was issued, you don't need to have the idea of memory in the name of the document. So I would just say "Document" or "Documents" issued to commemorate ...
I hope that is helpful.