I stumbled upon this blog of historic Portland photos and a number of them show these 2-aspect signal heads. This particular photos shows several of them along with what look like cast iron humps in the road. I know some early detection used the car horn as a form of changing the signals...perhaps this had something to do with it? I am also curious as to what brand the signals might be.
North on 39th Ave at Powell Blvd. - 1937
As it appears today