Appleby and Zummi-both of you are right in a sense
The history of "Series" finales is a confused one. First, there was a series finale only for the Virginia Slims from 1971 to 1976. By 1974 it was being promoted as part of a "Triple Crown", with Wimbledon and the US Open.
The problem for the women was that when the Slims circuit started, it was year-round on all surfaces. Starting in 1975, however, it was only indoors and ended in April.
Colgate stepped in to sponser a world-wide tour in 1977. Thus, from 1977 to 1982 the women had TWO "season ending" finales.
As the longer and larger circuit, the Colgate-Toyota finales had a bit more prestiege.
When Toyota pulled out we once again had a unified tour. Since the 1983 "season" was so short, the 83 Slims was an invitational rather than based on qualifying.
As Zummi said, the women switched dates in 1986, when there were 2 Slims finales. That was a good year to change as the Aussie wasn't held that year. The tour as we know it today more or less looks like 1987, except for less US indoor events.
Personally, I think some of these season ending events counted for a lot more than they do today. With 4 slams of near equal importance, the WTA finals just don't mean as much as they did in the 70's and 80's.
Hope I didn't add to any confusion