When did the American league begin allowing the manager more than one trip to the mound without changing pitch

I remember back in the 60's and may be early 70's a manager could only visit a pitcher once in the entire game. The second visit the pitcher had to be pulled. The pitching coach never did make a visit back then as well. It was always the manager. this was for both leagues. I have looked all over the internet tonight to find what year that changed, but to no avail.