The "Tixier" graffiti that Reno Davis (George Peppard passes twice on the riverbank refers to the real-life 'Jean-Louis Tixier-Vignancour', who contested the 1965 French presidential election for the far-right--which would have made him a popular choice for some of the movie's protagonists.
Oddly, the film leaves out the detail which explains the title - the all-important list of names which George Peppard 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 the Patience Collier character is seen laying out Tarot cards has no resonance for the later scenes.
Stanley Ellin's novel is specifically 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 were the basis for Frederick Forsyth's famous novel "The Day Of The Jackal" and the 1973 film version thereof. 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.