The 1969-70 season was the third after expansion prior to the 1967-68 season. The NHL placed all of the expansion teams into the new West Division and the original 6 made up the East.
St. Louis represented the West in the Stanley Cup Finals those first 3 years, getting swept by the East representative each time. The Blues have yet to win a Stanley Cup Final game.
For the 1970-71 season, Buffalo and Vancouver joined the league and were placed in the East. Chicago was moved to the West to even out the divisions. Philadelphia was the first expansion team to win the Cup, in 1974.