This is exactly what I was thinking. I have seen that exact socket on a bunch of poles in my grandmothers shed years ago. They were various tools such as brooms and mops.I was also thinking a mop of some kind. Looks kind of home-made, but might have been mass-produced by hand. And I don't think that's a bulb socket although it looks kind of like one. I just think it's a socket designed to fit a standard pole. That would be somewhat smaller than the standard medium screw lamp base. Appears to be made of copper and soldered together... they don't make 'em like that anymore.