I would bet the NPT bung fitting was installed dry by the looks of things. If it was mine, I would take a shot at an external seal that may or may not last, but you would need to drain all fuel, clean..really clean the area to be free of paint dirt and fuel all around the fitting and the rivet tails. Stainless wire brushes, ect and then final clean with MEK or Coleman lantern fuel (my favorite). I would then apply a brush coat of PR1422A 1/2 sealant over the area, let cure, and then apply a second coat. After you apply first coat, stick the Proseal in your freezer and it will still be usable for the second coat a few hours later. If the rest of your tanks were put together with dry areas, you may unfortunately find other leaks down the road that will require tank removal. I just did a re-seal on an RV-8 tank that had the bungs installed dry and no leaks after years of flying, but other areas leaked requiring removal.
8A7 / Advance NC