Is this pipe thread or inverted flare?
WILLIAM J This is not a tapered thread. It is 3/8-24 to 1/2-20, I believe. They are standard SCUBA sizes for low pressure regulator connections. These are standard SAE bolt sizes, both sides fine thread, not the most common thread which is more coarse. They are sealed by o-rings on both sides.
CHRIS K If you look closely at the picture, you can see an oring on the 3/8 side and a smooth seat for another oring on the 1/2 side. These are machine thread (i.e. the threads DO NOT taper) and are air/water tight via the orings.
BOB H this is the inverted flare. a regular hose end (3/8 Male), and large first stage port on some regulators (1/2 female).