Beer stimulates milk production (no, seriously - my wife is a nurse and occassionally has to teach breasfeeding to first time moms). Mothers who are having trouble nursing are encouraged to have a bit of beer. As long as they don't nurse the baby within the hour, the tiny bit of alcohol in ~4oz of beer will be metabolized - and they can always pump and toss the milk if still concerned.
What's really surprising about that add from the 70's is that it didn't come with a coupon for a case of Marlboro's too