Having taken apart quite a few of these engines in the past, I can tell you that it is just another oil drain plug for the transmission fluid. The fluid actually lubricates two chambers. One chamber is on the right side of the engine where the 2 speed transmission is located. This is where you check the oil level with the bike on flat/level ground. When the engine is running, some of the fluid is transferred through holes between the chambers so that the internal gear shafts that turn the sprocket are lubricated. The nut you are inquiring about is simply a drain plug to get the remaining oil of that chamber. Hope this helps. Bill