The cone shaped nut, which probably has a thin hex disc, goes against the prop. The locking nut is one which goes over the outside of the prop nut. The locking nut looks bigger but if you look inside you will see it has very few turns of thread, most of it is smooth, to go over the cone of the prop nut. The cone shaped nut may seem less, but look inside and you will see it is threaded all the way, it is the true prop nut.

Harry