confirmed, without the adapter, the thread is 5/16-24 for porter, Brooks flow meters , and 3/8-24 for matheson, aalborg, cole palmer, gilmount flow meters.
if need adapter, pm me now, let me know if you need one or two pieces.
I don't have the oring, but I guess it is easy to find the similar size around your house.
Porter flow meter, the black fitting is the original, and the brass fitting is 5/16-24 to 1/8 male npt, can be used as adapter.
The port is 3/8-24 thread on the matheson flow meter after taking out the adapter.