Someone I know (not me, honest) received this item in the post today. No return address or indication of the sender or vendor.
I suspect it might be connected with a “brushing” scam, in which case other than the usual checks of account hacks and suchlike there’s probably little we can do.
I am however intrigued as to what it actually is. The round end has a splined socket with a keyway and looks of a broadly similar size to a class 1 tractor PTO, so I might try it for fit tomorrow, though I’d not trust such a low quality piece of cast aluminium to drive anything off a PTO (a PTO-driven fan, perhaps?).