Rushmore is a manufacturer of Twin-Flat engines, four-stroke and geared, for light aircraft. It's Rushmore Motors Ltd.

“Three views of the Rushmore engine”

From Appendix 9: In the main text there are three photographs of this engine that are very small. Now we have one of them that is a little larger, although it is still somewhat diffuse.

“Rushmore boxer engine, RM-55”

-This is the RM-55 model engine giving 30 kW of power.