That's the Acerbis 1.1 gallon auxilliary number-plate gas tank. As designed, it's supposed to mount to the fork tubes. The problem is that the brake hose won't clear with it mounted that way, nor can you mount a headlight.
It took a bunch of time to fabricate that bracket out of tubing, then brazing or welding every joint. It's a one-off that only fits that ATK triple clamp. I'm not entirely happy with it but it was the only way to add the needed range to the bike.