The P2 takes 3/8-16 bolts. You don't need anything special from Predator. Headless bolts (set screws) work well, and you can use short ones to easily fine-tune the weight. The ones I have require a 3/16" hex wrench (key) for installation, and, depending on the depth of the tapped hole in the butt, you may need a fairly long wrench to screw the bolt(s) all the way in. Google headless 3/8-16 bolts or set screws to find many suppliers, and some billiard supply houses carry them.
Some flat-headed 3/8-16 bolts may also work if the head doesn't interfere with the rubber bumper.