Megan was wearing a bikini in the debut of Mad Men season 6 and it just looked off to me, it looks more like a cusper or 1970s and beyond to me. Dons trunks were fine, but Megan, not so much. Maybe it's the way the ties on the sides make it look more skimpy. While the bikini did show more skin in the1960s, the waist lines weren't all that low until very late in the decade. The leg lines are right so maybe it is closer to 1969.
Here's a video from 1968 featuring Anne Randall for Coppertone with Army Archard as interviewer.
1967 Bill Blass for Roxanne Swimsuit in stretch Dacron Knit $28.00, worn by Marilyn Tindall
Rikki for Sport Trio modeled by Marilyn Tindall
And check our previous post on vintage swimwear styles from the 1960s, I'd say 1969 and beyond for Megans bikini swimsuit.