It's interesting to think how different this film would have been from the colorful Pop Art fantasy of Mario Bava. Holt's directorial style seemed to favor stark, almost noir-ish black and white compositions, which would have cast the King of Crime in a very different, though doubtless equally flattering, style. How much of that film would have felt drawn directly from the pages of a fumetti is open to discussion. What isn't debatable is that it's a damn shame this film was never produced--in my universe, there is quite simply NEVER enough Diabolik!
Above is an ad that ran in 1966 in which producer Tonino Cervi seeks an exciting new actor with a handsome, masculine face, clear eyes, and an athletic build who is no less than 6 feet tall to play dashing criminal mastermind Diabolik in this film. De Laurentiis and Bava would find just such an actor in John Philip Law not too long after this advertisement ran.