Mystery Comics Digest was published for 26 issues, from 1972 to 1975.
All reprinted stories from three of Gold Key's mystery/suspense/fantasy/science fiction anthologies: Ripley's Believe It or Not!, Boris Karloff's Tales of Mystery, and Twilight Zone, in a three-issue rotation.
Some of these would introduce characters who appeared in Don Glut's titles at Gold Key: Dagar, Doctor Spektor, and Tragg.
The first two Duroc stories were originally intended to feature Dagar.
[1] Also, several of the creatures that Dr. Spektor fought appeared here first, including Ra-ka-tep the mummy (#1), Count Wulfstein the werewolf (#2), Simbar the werelion (#3), Baron Tibor the vampire (#4), and the Lurker in the Swamp (#7).