The series is apparently set during the administration of Ulysses S. Grant. However many of the years and dates mentioned during the series conflict with this setting.
The fashions worn in the series reflected the 1960s filming era rather than its 1880s setting. For example, a proper woman such as Victoria Barkley would have never worn pants and nearly all of the men's clothing is period inappropriate.
The majority of the weapons used in the series were manufactured far later than its mid-1870s setting. In fact, many are from the 1890s and after the turn of the 20th century.