This film was based on an episode entitled "Line Camp" in Sam Peckinpah's TV series The Westerner (1960). The episode was also written and directed by this films writer and director, Tom Gries [See: The Westerner: Line Camp (1960))].
During the fight between Will Penny and the Quint family, one of the brothers throws a knife into Penny's chest. A wire attached to the knife a very obvious.