The stock bearing plate is mandatory for wood stocks but not used on synthetic stocks. This plate slips over the receiver stud before you install the wood stock. It holds the stock a fraction off the receiver and prevents the stock from chipping or splitting under recoil. The 12ga fits 870, 1100 and 1187 Shotguns. The 20ga fits 870, 1100, 11-87 shotguns and 7600, 7400 and 750 Remington Rifles. Select the correct one from the drop down list. This items retails for $4.99 from Remington.
Note if you are installing wood stocks on a gun that had synthetic stock you will need this plate if it did not come with the stock set. DO NOT INSTALL A WOOD STOCK ON THE GUN WITHOUT THIS PLATE - If you do the stock can chip or even split under recoil. Don't damage a stock worth a couple hundred dollars for want of a $4.75 part.