Limited Collectors' Edition C-21 (Summer 1973) was the first to actually be released under that title, with that numbering. It was the first of three Shazam! issues, reprinting stories of Captain Marvel and the Marvel Family, Mary Marvel and Captain Marvel, Jr., the other two being C-27 (1974) and C-35 (April/May 1975).
The stories reprinted were all originally published by Fawcett Comics. DC had just started licensing the use of Captain Marvel, and would shortly purchase the character outright.
The third and last of the Shazam! issues featured a photo cover of the actor who played Captain Marvel on the live action Saturday morning tv show.