Eric,
I took off the large nut about a year ago. My problem was the thumbscrew was leaking oil a bit. BTW, there is an o-ring on it that can only be accessed by removing the nut. The o-ring was only about three years old but always had a small drip. I went ahead and replaced it with a fractional sized o-ring that was a few thousands thicker than the metric. Since it is a static seal, it worked perfectly. Best to replace it while the nut is off.
I did not find any seal between the nut and the body and had the same question as you. I believe that there is NO extra seal. If you look at the face of the nut that goes into the pump, there is a very thin protrusion that I believe seals on the aluminum pump body. So you have steel mashed into aluminum. Same sort of arrangement as a seal ring. The narrow protrusion on the nut cannot have any damage in order to work though. If there is a ding in it, the oil will leak past there. I didn't put any external seal on and mine doesn't leak a drop. You could always call Pacific Fuel Injection and briefly ask them to confirm.
