TPO had ridden it with the g/box pivot hanging out, thus odd wear to chain and sprockets. G/box sprocket issue. The one fitted has splines and flutes of the same dimmensions, and the replacement has narrow flutes, about 1/2 the width of the splines. So the question is.....any ideas as to what machine the gearbox orriginaly would have been fitted to. It is marked STD.