Fuel Tank Fitting (Alloy) inc. Nut & 2 Fibre Washers. Use this if you want to convert a "top-feed" tank to a bottom feeder.
By the way, the trick to mounting them is to run a thin piece of wire down through the fuel filler neck, and out the hole you've drilled in the side of the tank. Slide the fitting down the wire until you can get your grubby fingers on it (don't forget the fibre washer for the inside, too!), then put the other fibre washer on the outside, wind on the nut, and Bob's your auntie's brother!