I recently bought a Mossberg 500 Cruiser and was about to purchase a forearm (front) pistol grip when i read under one description for a Tacstar that it may not fit on models after 2006. I checked a Pachmyer version and it said the same thing. I checked for a "castle nut" but i'm not real sure on what it is i'm supposed to be looking for. Anyone know offhand what i'm talking about? Any help would be appreciated before i go and spend money on something that may not fit?