![](http://www.employees.org/~beetle/guns/hp1911/right.JPG)
Ball cut.
![](https://i.pinimg.com/originals/f2/a0/ac/f2a0acdc1c8da8d71f48cbac24b97503.jpg)
Slant cut that replaced it.
It called a ball cut because that was the shape of the cutting tool; I used to know why Colt switched , do not remember exactly but I beliee the slant cut was quicker and therefore cheaper.
Used to know what year they switched , do not now, but all Colt ball cut guns had the fine blue finish so it was early.