I'm going with "lightning-rod finial" (a.k.a. top), for two reasons. (1) Although it is the shortest one I've seen, it has the same triangular-bladed form as the one shown in the photos below. (2) It is thick brass which is oddly twisted without showing any signs of being hammered-on or gripped with tongs/pliars to do the twisting. It actually looks like it got lightning-struck.