This is a Nordic Components Shotgun Magazine Tube Components, model NUT-RM-12-00.
The MXT extension nut allows for installation of a Nordic Components MXT 12ga shotgun extension tube kit. The nut replaces the factory magazine cap and is required for installation of the extension tube kit.
Features:
Remington 870/1100/11-87/V3
Replaces Factory Magazine Cap
Required for MXT Extension