Here’s an ad from the April 29 1936 edition of the Toronto Daily Star that offered a free guide with every Parker Pen.

After reading this, I wanted to know: who was Princess Rostislav, and did she actually write a “Guide to Correct Social Correspondence”?
It looks like “Princess Rostislav” might refer to the first wife of Prince Rostislav Alexandrovich (1902-1978), who was born Princess Alexandra Pavlovna Galitzine. The prince and princess were married in 1928 in Chicago, and were divorced in 1944. The National Portrait Gallery has photographs of the princess that were taken in 1922, when she was 17.
I have no idea whether this is the princess in question, or whether she actually did write a guide to correct social correspondence. I could find no reference to the guide anywhere, except in a reference from another ad in a Vancouver newspaper. If she had actually written such a guide, she would have had plenty of time to revise and republish it, as she passed away in 2006 at the age of 101.
Prince Rostislav, a nephew of Nicholas II, the last Czar of Russia, probably considered himself lucky to be alive and married to anyone: he escaped being assassinated by the Bolsheviks when German troops freed him in 1918. After he and the princess divorced, he went on to marry Alice Eilken and then Hedwig Maria Gertrud Eva von Chappuis; she married Lester Armour, who was almost certainly not a prince (at least not literally).