Hmm...I remain unconvinced that it is a decorative fence spike. A tool perhaps, as those implements are always popping up resembling their military cousins. The crosspiece looks too much like a guard to me, but because we are guessing...we are guessing!
The reinforced/raised ridge does resemble a feature found on many bayonets. I've seen French sword blades with this odd feature as well. I'm reminded of the way many of the Span colonial iron pieces were secured to hilts/hafts/shafts being quite different than their contemporaries. Early Span trade axes were axe heads sandwiched in between a split haft. Espada often had odd hilt configurations, with the tang again sandwiched in between slats of wood. My point being, perhaps IF this were a pole arm, it never had an open socket, but could have been secured to the pole via straps? I know, going fishing here. I have a feeling this is going to be one of those items we never get clarity on-