When did the porn biz start using "xxx"?

[Rubber Soul]

I always see the three x's on store windows or on boxes of videos. I never really put any thought to it. It's like, why do bears fuck in the woods? They just do. But I saw the three x's right now in a movie(Blast From The Past) and I began to wonder when did they start using them and what were they intended to to stand for.