This adapter came with the Shimano M465 disk brakes that are standard on the Giant Anthem X3 2010 in Australia. It is used to run a 160mm rotor on the rear. It can also be used to run a 180mm rotor on the front.
The adapter weighs 18g.
The bolts required are M6 x 18mm with 1.0mm thread pitch.
The XTR IS to post mount adapter is significantly lighter at about 12g - Shimano MA-90 Disc Brake Mounting Bracket