Recently this "thingy" came into my possession ..... I'm thinking it may be a seatpost for a tandem with an adjustable clamp for stoker bars (square profile and underslung). But I'm just guessing ... and how to attach a seat to the short side extension? Or is the underslung clamp perhaps from something else and it's just an odd seatpost with extra setback? I haven't measured the centre column but it seems around 26.0 to about 26.6mm in diameter.
BTW, if you need this lemme know as I am willing to trade it for some other bike parts of interest to me.