I looked at my box that parts came in. The black plastic cap is part number AL 113086. I have another parts bag with part number L 80418 on it. I think that is the threaded filler neck. There is a rubber o-ring under filler neck (which threads into plastic tank nozzle). The o-ring seals between metal neck and plastic tank. My tractor is a 2012 5065e. I doubt the tank changed. The o-ring is part number L 112337.