Going to put at least one more entree on my table as there is some more time to fill.
These engines are not really rare at all, but they are still rather uncommon, and exceptionally unique. They have a boiler made of plastic, the water in the boiler functions as the heating element, and the engine is re-configurable into three different engine types.
I'm speaking of the K. J. Miller SE-100 Junior Engineer of course .... and here are a semi "Rare Pair" of these unusual engine in the Beam and Marine configurations: