Sam Elliott, Tom Selleck, and Jeff Osterhage had such a good time doing this project together, they reunited on another 'Louis L'Amour' story, The Shadow Riders (1982), years later.
After the dummy gets shot in the jail cell, Tyrel and Orrin run out to catch the shooter. When the camera goes back to the prisoner in the other cell, you can see Tyrel moving around behind him.
When Tom, Cap, Orrin, and Tyrel ride into Purgatorie, you can see a large telephone pole with modern transformers on it, in the background over the buildings.