Hmmm. I am not so sure about that Ralph. I asked Marvel Schleber themselves and they came back with:
I further asked what was the correct male thread to connect into that and they came back with a link to
this page on a German database of standards.
It clearly says and shows a diagram that has the straight thread of the female NPST mating with the 1.47 degree TAPERED male thread of the NPT. So it seems that fittings such as AN816 (AN822 or AN823) -6D would be compatible as they have the NPT 1/4” 18 thread. As the nominal dimension of a tapered thread is taken half way down the run of threads, I would expect the fitting to ‘bind’ in the inlet when threaded about half-way in. This is
probably the “dryseal” feature referred to and coincides with my own experience of test fitting an AN816
However, according to this information, the fuel pump fittings as sold by Van’s and ACS (among others) with a STRAIGHT UNF 9/16” 18 thread will fit because the two straight threads will mesh perfectly. However, the “dryseal” feature won’t operate. A proper seal therefore would depend on the O ring and some sealant.
As always, I am happy to be corrected on this