Judging from the first advance promo photo of the last run of “Mad Men” episodes, the costumers are taking their cues not so much from the 1960s/70s as they are from 60s/70s era “Legion of Super Hero” comic book. The first thing I thought of when I saw Jessica Pare (a.k.a. Megan, a.k.a. the ex- Mrs. Draper #2) was 'It’s Phantom Girl. Meanwhile, Sally certainly looks like she’s rocking a dress that Duo Damsel wouldn’t have seemed out of place wearing.
Having been born in May of '69, I don’t quite remember the early 1970s. But did girls / women really dress that way back then? I remember bell bottoms, clogs, and quasi-hippie fashions, but those clothes look a little strange.