I just replaced my 870's factory stock (Remington told me that 1187/870 use the same stock).
The holes on the recoil pad are really deceiving, I actually used a standard size phillips (or maybe a little bigger) screwdriver to unscrew it.
Apply soap or non-corrosive lubricant around the shaft of the screwdriver like oneounce said, and unscrew counter clockwise. Don't be afraid to use a little force.