This is a Allen Swivel, model 14430.
This swivel set comes with 2 Allen Magnum Swivels that fit most pump action and semi-automatic shotguns without hardware installed" and the swivels are designed for slings up to 1" wide. Contains the necessary hardware to install swivels on most pump action and semi-automatic shotguns. A threaded stud designed for installing into wood or synthetic stocks is included for the butt stock attachment. A rotating stud that requires drilling the factory magazine tube cap is included for attaching to the fore end.
Features:
Includes all hardware
Can be used with wood or synthetic stocks