Russell, it does have an o-ring and I would tend to agree with you since I know the seal on a NPT is on the thread face. But you can see a lean to the thread that the straight thread does not have?
HopCar, I have looked online for hours and it is confusing. Now I did find a 1.5" NPT on go two marine. 71 bucks just for a cap. I can buy a complete deck fitting for 1/2 that price. That makes me think it may be the one I need. Since it seven times the price of other caps??