[acorange]

What Colonel Klink said... The plug was called a "vari-weight plug". They can be found from time to time on e-bay. They fit in the mag tube and the buttstock . I presently have 5 / 870 traps and they ALL have extra ballast in them. A mercury or spring recoil reducer also works. Just wrap it in tape on the ends so it just fits in the stock hole or mag tube so it does not rattle around. On the composite stocks all that I have found that works is filling the cavity with shot. H&R also made a 7/8" steel tube that can be cut to length for recoil reduction. These also can be found on e-bay.
A 870 with out weight in it will wear you out with recoil unless you are shooting very lite loads.