Oddly, the film leaves out the detail which explains the title - the all-important list of names which Reno gets hold of is, in the novel, in code, which is broken via a knowledge of Tarot cards. In the film, it's just a list of names, so the brief moment where Gabrielle is seen laying out Tarot cards has no resonance for the later scenes.
Eva Renzi started filming in early 1967, but was fired after she said to John Guillermin: "Your last film was so brilliant. I don't understand why you're doing this piece of shit now."
The "Tixier" graffiti that Reno passes twice on the riverbank refers to Jean-Louis Tixier-Vignancour, who contested the 1965 French presidential election for the far-right, that fact which would have made him a popular choice for some of the movie's protagonists.
Stanley Ellin's source novel is set in 1961, and the far-right organization plotting to seize control of France is specifically identified as the "Secret Army Organization", or OAS, a real-life group formed by military officers after Charles de Gaulle granted independence to the former French colony of Algeria. The OAS made several attempts on de Gaulle's life, without success. These attempts are the basis for Frederick Forsyth's novel "The Day of the Jackal" and the subsequent film version (Il giorno dello sciacallo (1973)). Most of the leaders of the OAS had been arrested or killed by 1963, after which the group fell apart. This explains why the conspirators in this film are not specifically linked to it, given the updating of the story.