For anyone interested, here are some links to complete United Airline system timetables:

From June 1, 1972:

http://www.departedflights.com/UA060172intro.html

From June 11, 1976:

http://www.departedflights.com/UA061176intro.html

From August 1, 1982:

http://www.departedflights.com/UA080182intro.html