As they all said its a CAM (camber inducer), but since the is a lot of velcro consider this:- Some older sailes eg: KA Reactor had extenal cams that velcroed around the mast in the cut out area - early 90's - slip over batten end (would have velcro at this end) then wrap around mast.
yer it's off one of those old sails, raf sail not raf sail kind of thing, the funny thing is, no one ever used them, like the old twin cam Pryde sails.