Just a little history on the Wankel engine. NSU (the German car company) bought the rights to build the Wankel and sold on the rights to Graupner allowing them to make model sized Wankels. Graupner in turn contracted OS to actually make them so they were usually boxed as a Graupner-OS.

But because they have such complicated (read odd shaped) machining, OS had to invest in buying specialised CNC machines. So the Wankels were the first CNC made model engines. A couple of years later OS brought out their new line of 2 strokes and naturally used their CNC machines to make them. The first CNC made 2 stroke was the 40SR which was a rear induction pylon engine. This engine (well, the crankcase anyway ) later morphed into the more familiar 40FSR for sport flying and presumably the "F" stands for Front induction. That's why the FSR engines only have SR cast under the exhaust port because it's the same casting as the original SR pylon engine.