I recently saw a photo of a 1903 Ellensburg semi-pro team in almost identical button-down p-coats. A few of the nine worked for the Northern Pacific Railroad so I had assumed they were stevedore uniforms they stole/borrowed for the photo. This totally realigns my thinking. It really does seem to be the popular style of overcoat at the time. Probably, heavy, thick wool the weatherproofing of its day.