RE: Is this a stock Cox backplate or modified?
It must have been for a speed model, the owner figured he'd pick up another 5 mph by slimming it down to the max..
I see clean metal around the perimeter, though there is some mottling or something around the base of the fuel nipples. It would be dead easy to mask the nipples with fuel tubing, and plug the NV hole etc., to hit it with a sand or bead blaster - and that is the sort of equipment that is not uncommon to have access to. Just a thought..? That grey appearance of the backplate perimeter and somewhat normal look of the fuel nipples seems consistent with the appearance I'd expect - or maybe it is just an artifact of the photo reproduction.
No way it is stock Cox.
MJD