This is a Nordic Components Shotgun Magazine Tube Components known as the Mossberg, model NUT-MB-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:
Mossberg 590/590A1/835/930/935
Replaces Factory Magazine Cap
Required for MXT Extension