Alain Delon, like many French, had admired Machine-Gun Kelly (1958), one of the first Bronson's starring vehicles, and he invited Bronson to co-star with him in "Adieu, L'Ami".
For the French soundtrack, Charles Bronson's voice was purposely dubbed with a very strong American accent by director John Berry.
In the doctor's office there is a calendar that says Dec. 20th, Mercredi, so assuming it's accurate this was filmed in 1967.
Alain Delon originally wanted Richard Widmark as his co-star.