In October 1972, Random House printed The Secret of Terror Castle under their Windward Books name. One year later, in October 1973, The Mystery of the Stuttering Parrot appeared in the Windward line-up as well. These are both First Paperback Editions of these titles.
Windward Books was a subsidiary of Random House that produced a "series" of large-sized paperback favorites for children. I say "series" because each book brought out under Windward had a number printed on the spine. Terror Castle was the 20th book in this series and thus has a "20" on the lower spine.Stuttering Parrot was #23 in the Windward series. These are the only two Three Investigators titles that appeared in this series but Robert Arthur enthusiasts will be interested to know that the following four children's anthologies assembled by him also appear in the line-up: Mystery and More Mystery, Ghosts and More Ghosts, Thrillers and More Thrillers, and Spies and More Spies.
Both of The Three Investigators titles first appeared in this edition as a "Silverback." This is a name which refers to the reflective silver foil behind the cover art and title (these don't scan well, as you can see above). The covers of both titles feature cover artwork by Ed Vebell who painted the cover art for the original hardbound edition of these two titles back in 1964. Both titles also feature the Harry Kane internal illustrations from the original hardbound editions. It is interesting to note that Terror Castle contains the same revised Hitchcock Introduction that appeared one year earlier, in 1971, in the large-sized paperback Scholastic edition. In this revised intro, Hitchcock refers to his having previously introduced The Three Investigators case The Mystery of the Green Ghost. This no doubt confused Windward Books readers as Green Ghost never appeared in this particular line-up either before or after Terror Castle.
The printing history for these two Windward titles is unknown as Random House did not yet include this information on the copyright page. However, it is assumed they were not in print for very long because these titles in this format are not often found on the secondary market. The cover price for both titles was originally 95 cents. Occasionally one may find a 95 cent copy which has the silver foil on the insides of the covers but these appear to be fairly uncommon. Later Windward printings of both of these titles exist with a printed cover price of $1.45 and these books have a white background instead of the silver. These later white cover printings are extremely difficult to find and they are perhaps the rarest U.S. Three Investigators books of them all.