That may not be a drain plug.
Maybe the newer Yanmars have some sort of drain plug on the oil pan, but typically, Paul, you will have to extract the oil thru a small tube inserted thru the dip stick hole. This will require a special pump and tube designed for this. Make sure the engine oil is warmed up beforehand. Then do the filter.You should call Hunter, first. They may be able to explain the "phantom" drain plug that's on the engine. Good Luck.