RB engines were made by Borzecki (see) based on the cylinders of the Riedel boxer, the starter motor of the first German BMW and Junkers Jumo turbines.

"2RB motor with propeller, by the magnetic plate"

-They made 2 and 4 horizontally-opposed cylinder versions. The Author assumes that due to the total width of this engine it maintains the characteristic of having “square” cylinders, that is, equal stroke and bore.

"The 2RB is a double RB"