This is a Pachmayr Recoil Pad known as the Butt Pad, model 04414.
Slip-On Pads fit most stocks. Easy to install" these pads offer excellent recoil absorption. Pads come in two different sizes" and are available in black or brown" to complement any style stock. Pads add 3/4" to the length of pull.
Black rubber recoil pad
Slip-on butt pad
Fits small stock