My guess is that it has standard screw holes aligned vertically, one toward the top, one toward the bottom.

With a soft recoil pad, often the pad material "closes up" around the screw holes. Gently poking around with a Phillips screwdriver (or a nail or rod that has been ground down so it has a rounded tip that won't damage the pad material) can help find the holes without damaging the pad, and then once you find the holes, using lubrication on the shaft of the screwdriver (I know that sounds like a double entendre, but I am being very serious here) will help a lot. It keeps the pad from grabbing onto the screwdriver and trying to twist around the screwdriver shaft.