[TonyM]

Using your photo: The thicker part of the tube (the piece with the holes) is the threaded part. The fuel filler tube and washer are placed on the tank opening and the thicker part of the tube (packing nut) then threads into the fuel tank, holding the filler tube against the lead washer and the tank opening, sealing the tank. Here is a photo of the spanner that I used to twist the threaded tube/packing nut.