The Las Vegas, Nevada desert scenes were actually shot in the Almerian desert of Spain.
This picture was an international co-production of four countries: France, Italy, Spain and West Germany but not the USA even though it was shot there too and Americans Gary Lockwood, Lee J. Cobb and Jack Palance lead the cast.
According to Skorsky the amount of money in the armored car was seven and a half million dollars. This would equate to over $63.5M in 2022.
The title of the film's French source novel 'Les Hommes De Las Vegas' by André Lay literally translates into English as 'The Men of Las Vegas'. This movie was made and released within a year of the novel being published.
While the movie was filmed in the late 60s, the opening scene has a shootout in front of movie posters all from the early-mid 1950s: The Command (1954), Thunder Over the Plains (1953), So Big (1953), Phantom of the Rue Morgue (1954) and The Diamond Queen (1953).