Published in 1976 (no publisher listed), the Scene de Ballet Postcard Set was created for the New York City Ballet. This set of 10 cards (2 copies each of 5 images) came in a plain white envelope. These images were reprinted in The Betrayed Confidence and also appear in Amphigorey Again.
The first printing of these postcards have always been particularly difficult to find. I believe this is because they were originally sold at the Ballet and they were used by the people who bought them. I have occasionally seen partial sets offered, but complete sets in the envelope are rare. This was one of the last "holes" to be filled in my collection of first edition Gorey. Today, these images are readily available as a boxed set of notecards published by Pomegranate.