In 1981, Random House included a "Mystery Quiz" page in each of the eleven Three Investigators titles which saw their First Paperback Printings in that year. This includes the following nine titles which appeared in the Hitchcock Cover paperback format:
#13 Crooked Cat
#14 Coughing Dragon
#16 Nervous Lion
#17 Singing Serpent
#23 Invisible Dog
#25 Dancing Devil
#26 Headless Horse
#27 Magic Circle
#28 Deadly Double
Also included were the two new titles appearing that year in the new Keyhole Cover paperback format:
#31 Scar-Faced Beggar
#32 Blazing Cliffs
Some of the above titles saw a second and third printing in 1981 and some of them contain the bookmark as well. The Mystery Quiz page is just inside either the front or back cover of the book. The page is of a much heavier paper stock than the text. Two-thirds of the page (the inner portion) contains the Mystery Quiz which consists of three questions. The outermost one-third of the page is a detachable Three Investigators bookmark which features Stephen Marchesi's cover art from "Moaning Cave". The back of the bookmark will list either 26 or 32 Three Investigators titles. Books first printed after October 1981 have 32 titles listed because that is when the two new titles were released (#31 & #32). The bookmark is generally missing which, of course, helps an intact example command a premium price. The back of the Mystery Quiz portion contains the answers to the three questions posited on the front.
There are two variations of the Mystery Quiz page. The First Variation lists only 26 of the 32 titles available in the series on the back of the bookmark and the answers to the three questions are 1) Haunted Mirror; 2) Sinister Scarecrow; 3) Skeleton Island.
The Second Variation lists all 32 titles that were available on the back of the bookmark and has three different Quiz questions to which the answers are: 1) Magic Circle; 2) Vanishing Treasure; 3) Invisible Dog.
