In one scene, in Lawrence Murphy's house, there is a stuffed Chinese Ringneck Pheasant mounted on the wall. This game bird was introduced to the United States in 1881, the year Billy died, so it could not have been hunted in New Mexico at that time.
Sheriff Brady says that there is a large Hispanic community in Lincoln. The term "Hispanic" did not come into use until the 2nd half of the 1900s.