This is a Nordic Components Shotgun Magazine Tube Components known as the Beretta, model NUT-BR-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:
Benelli Nova/Supernova
Beretta 1301 Competition and Tactical, 391, A400 Xtreme
CZ 612/712/912
Weatherby SA-08