Richard,
The following information is from "The Illustrated Encyclopedia of Railroad Lighting Volume 2 The Railroad Signal Lamp" by Richard C. Barrett, with research by Joseph Gross and Richard C. Barrett. This book was published in 1999 by Railroad Research Publicaitons of Rochester, NY.
"Adams & Westlake MA-15
No. 29 Watt's Patent
Combination Caboose Tail Lamp
This lamp has a case which is permanently mounted to the side of the caboose. The light source itself is fully serviced from inside the caboose. The air for combustion comes from inside the car, while the products of combustion are exhausted to the outside through the top of the case. Two compartments, one on top of the other, show different signals when the lamp is placed in the compartment. The lower compartment was used when the train was on the main track and displays a red signal to the rear and a green signal ahead and to the side. When the train is sidetracked, the light is placed in the upper compartment thus displaying a green signal in all three directions. This lamp was shown in our catalogs from 1896 through 1907."
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Hope this is helpful.
Regards,
Bob Milhaupt
Illinois Railway Museum Volunteer