This looks like a 6.6 galon Acerbis tank for a Yamaha WRF 250 or 450.
It looks like it is sitting on top of the stock TW tank and the front part of the seat, this way he can still use the one and a half gallon of the stock tank. That could be very usefull if the about 12 gallon tank on the rear rack runs dry.
The tank bag is holding it all together with its belt.
But still a nice setup, most likely it is impossible to ride when all filled up.