When was a revolver with no hammer serial?

The first revolver without an external hammer was the "Derringer" style, which became popular in the mid-19th century. However, more specifically, the "double-action" revolvers, such as those produced by Colt and Smith & Wesson, began to feature designs without external hammers in the late 19th century. These designs allowed for quicker firing and easier concealment. The term "hammerless" is often used to describe these types of revolvers, which became more common in the early 20th century.