I had a leak on top of my primary fuel filter (the spin on one). I first thought it was the injector return hose leaking. I fixed that, and afterwards noticed that a very small drip of fuel was coming from what looks like a 7/8" nut on top of the filter housing. Is this is a plug, or a bolt that goes all the way through, and is there is a gasket that should go under it. Also, any part numbers would help.

Thanks,