牛仔裤大兜里的小口袋是什么用途?为啥现在少见了 | 有趣的制造
看来只有自己动手查资料了。
The small pocket on the right front side is called a watch pocket, since it was intended to store a pocket watch — a typical possession during the late 1800s.
https://www.levistrauss.com/2017/01/12/pockets-full-history/