This might be waaay off, but without any measurements or actual comparison, I'm really just guessing...

buuut, following the lines of your supposition, I wonder if it could be a non-OEM backrest support bracket; maybe something someone's fab'bed up at home to change their Backrest angle? Do those 4 small dia holes match up with the holes on the underside of the seat pan? And if they do, would the upright bit slide into the cavity going up thru the seat foam from the pan/base toward the outlet hole in the seat cover at anywhere near the right angle for it to have anything to do with the Driver's Backrest? :dontknow:
Juuust takin' a punt on what it might be?! AFAI Can Recall, the OEM Backrest Bracket has a couple of bends in it to shape its profile to match the step in the seat pan below the Driver's Lumbar Support Ridge, and that one looks as tho it'd bridge flat across that step instead of bending to match the step's profile. Whatever, it certainly looks 'fabricated' by someone rather than being 'factory manufactured'?!
Innnterrresting, but not funny?? Anyone else with other ideas?? I'm happy to be edified... :thumbup: