In one of the episodes (or maybe more) Donna's mother is wearing the 90's-2000 version of the shag haircut - to the shoulders and trimmed/combed toward the neck. In the 70's the "real shag" was in style - short on the sides, longer in the back - like the Brady mom.
It is a very big assumption that absolutely no one in the 70's had that haircut, although it is possible. Maybe she was a trend setter and had a very innovative hair stylist :)