Thanks,
@parallelmotion. Can I use the picture in my article?
The only evidence I currently have as to who made these three engines is circumstantial. Can you judge an engine by the company it keeps?
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The Hero engine, which is also in the Smithsonian, is likely the engine in the front, just to the right in this picture.

The designer/manufacture of Maxwell's engine also produced a fire engine. It has the same flywheel and boiler shown in the horizontal above. I haven't found a picture of that engine yet.