Ive got an 870P that had the supercell installed from the factory on a walnut (not laminated, solid walnut) stock. The fit is "almost" perfect, the stock fits completely flush on the bottom, and is larger than the pad by about ~1mm on the top curve. It looks great, but I do not know how much if any fitting was done at the factory, but being a police magnum in walnut, i wouldnt have paid for it had the fit not been really really good.
FWIW, the fit on the pad is actually a little tighter than the fit on the receiver, which is still very good IMO