...Strange headed bolt?
This gets my vote. It may not be a fillister screw but it's gotta be something that the prop guys just had lying around to plug (if it's there) the hole underneath. I don't see them going through the trouble of getting a chunk of aluminium, threading the hole in the saber, then drilling and tapping a hole in the chunk and then securing that to a saber with a set screw or something. It would've made more sense, to me, that they just found something on hand to plug it in with. :unsure
That, or it's a wart.