At one time these were rather common.
The four brackets were for rotating the marker lamp to show the required light color in the direction ordered by the rules.
For example, a train taking a siding would show an amber or green light (which color shows depends on the railroad and its rules) on the side where a following train would pass on the main track. The other marker remained red.
The track-side marker would be changed to red again when the train moved out onto the main line.
Attached, 1953 rules governing use of class and marker lights on the B&O. While a steam loco and passenger car are shown, the same rules applied to diesels and cabooses.
Later designs for class and marker lamps allowed turning the lamp body to index points on its frame to change the light color. The rules also include notations on moving color roundels in electric markers and class lights to change the aspect.
S. Islander
