It's generally accepted that names like
Susan,
Barbra and
Linda were not actually being given to boys, but that some of the
Susan etc records are mis-coded as male. You only see it at times when the name was very popular because that's when it generated enough mis-coded records to show up on the charts.
I'd be interested to be proven wrong, but I've yet to hear of a single man with a feminine name from that time period.
http://christineseaforthfinch.blogspot.com/