I believe it was in the mid 50's. Primarily in magnum calibers, but I have also seen them in other "non-magnum" caliber hammers. Doesn't seem to be any rhyme or reason, or regular, consistent fitment of them. Some of the magnum revolver hammers have them and some don't.
I'm not absolutely sure about the reason for them either, other than reducing stress on the hammer, frame and primer. Perhaps someone will chime in soon that actually knows what they are talking about.