When Alfred Hitchcock died in 1980, Random House had a dilemma on their hands. Should they continue The Three Investigators series with Hitchcock or find a replacement for him? Evidently, they seriously considered their options for some time as this was the very first and the very last year in which no new Three Investigators titles saw publication in the whole 24-year history of the series. In 1981, with the publication of #31 Scar-Faced Beggar and #32 Blazing Cliffs, readers were introduced to Hitchcock's replacement: the fictional Hector Sebastian. Of course, because Hitchcock was no longer a feature of the stories, his silhouette could no longer be used on the spines and covers. Random House came up with the keyhole shape to represent the books which do not feature Hitchcock. The keyhole symbol can be found at the top of the spine and in the top right corner of the front covers. Note: The scan to the right shows the spines of a Marchesi paperback, a Hitchcock Cover paperback and a Keyhole Cover paperback, in that order.
It should be noted (as detailed in the interview with Dennis Lynds) that the Hitchcock estate had no problem with Random House continuing to feature Mr. Hitchcock in future stories. It was the decision of Random House to develop a new "Host", most likely in order to avoid paying the large percentage to Hitchcock's estate. Within three years of the entire series being revised to include Hector Sebastian, the original Three Investigators series was out-of-print. In Germany, where they opted to stick with Hitchcock, the series has thrived and grown to over 100 titles.
Beginning in 1981, all of the new titles (beginning with #31) featured Hector Sebastian and had the keyhole symbol on the books. Titles #1 - #30 continued to be printed in either the Marchesi Cover or Hitchcock Cover paperback formats up until 1984. In 1984 and 1985, Random House revised the first 30 titles, replacing Hitchcock with Sebastian in all of them except for the very first book Terror Castle, which features the fictional movie producer, Reginald Clarke. None of the Keyhole Cover books contain internal illustrations. This changeover of the first 30 titles also constitutes a new, or revised, paperback edition. It is possible for a collector to build an entire set of all 43 original Three Investigators titles in paperback with the keyhole symbol to achieve a uniform-looking set. The only other way to build a completely uniform-looking set of all 43 titles is to combine hardbound trade editions #1 - #28 with GLB books #29 - #43. You can view scans of all 43 Keyhole Cover paperbacks by going to the Original Series page on this site.
Here is the order, by year, in which the Keyhole Cover paperbacks were published: