Limited Collectors' Edition C-39 (Oct/Nov 75) was the first of two issues to use that theme. I had both of them, and really loved them. I was just getting into super hero comics when this came out, and it introduced me to a lot of villains I had not yet seen. This issue reprints the origins of The Joker, Lex Luthor, Captain Cold, Sivana and Terra-Man, in stories ranging from the 40s to the 70s.
The centre spread showcases a huge number of villains. Even as a kid, I noticed that it was the work of different artists, and deduced that the Flash villains who make up the core of the picture were all by the same person. Over the years I found the various comics in which the other illustrations are taken from.
C-45 (June/July 1976) bills More Secret Origin of Super-Villains, and includes reprints of the origin of Catwoman, Mirror Master, Mr. Mxyzptlk and the Cheetah, in stories from the 40s and 60s.