In the Spring 2012 issue of the British magazine Illustration, Edward Bradford wrote an informative article about the postcards created by Edward Gorey. Mr. Bradford, the official Edward Gorey bibliographer, follows Mr. Gorey's postcard images from The Broken Spoke in 1976 through the end of his career.
The Broken Spoke (see my 3 postings regarding this publication from August 2009) was first published in the June 14, 1976 issue of Sports Illustrated as Cycling Cards. Shown in the magazine are are a handful of postcard images all relating to the sport of cycling, supposedly created by Dogear Wyrde (a Gorey pseudonym) and collected by Mr. Gorey.
According to Mr. Bradford, Mr. Gorey was encouraged to publish these images in book form by Andreas Brown, owner of The Gotham Book Mart in New York City. The book was published in October 1976 with a total of thirty postcard images. In his article, Mr. Bradford says it is unclear if the additional book images were created specifically for the book, or if there were simply too many images for the article. Since the original artwork from The Broken Spoke was sold by Mr. Gorey through Gotham, it is not in the Gorey archives to reference to answer the question.
After reading Mr. Bradford's article, I obtained a copy of the magazine to see which images were shown in the article. I own two pieces of original artwork from The Broken Spoke, and found out that one of my pieces, Bats & Bicycles is not included in the article. When creating artwork for his "A Collection" books, Mr. Gorey would put the date he began and completed each piece of artwork on the back of each drawing. Looking at the back of the Bats piece of art, I found the dates 8/20/76 8/20/76, so we now know that additional artwork was created specifically for the book...And we know what Edward Gorey was drawing on August 20, 1976!