It's a watchpocket, originally for men who wore pocket watches and needed a protective place to store them.
这其实是一个表袋，最初是为有怀表的人设计的，怀表放在这里面刚刚好。
2
The Levi Strauss blog confirms their conclusion: "the first blue jeans had four pockets-only one in back and, in the front, two plus the small, watchpocket."