DKW racing engine of the 1930s 

I remember reading about a DKW racing motorcycle engine with three cylinders but one combustion chamber.  In essence, the two outside cylinders were mounted vertically and formed a Twingle or split-single unit, while the middle cylinder was mounted horizontally (facing forward in the frame), had a larger bore and a smaller stroke, and was essentially a supercharger for the other cylinders.

Does anyone have any documentation for this engine?  I think the motorcycles it powered were successful racers at the time.