Ennio Morricone's musical theme "Chi Mai" was composed for Maddalena (1971). The song was completely re-recorded in 1978 for the single "Disco '78" with heavier drums and in a different key. After hearing this version on the radio, Jean-Paul Belmondo was so impressed that he suggested it should be put into the soundtrack of his film. Ultimately, the record of this musical score was not only more successful than its initial release but came to be one of Morricone's bestselling work.
The trivia item below may give away important plot points.
At the end, Beaumont is killed just before getting in the helicopter. Georges Lautner shot another version where Beaumont escapes in the helicopter, but chose with Jean-Paul Belmondo (against the wish of the producers) to take the other one.