This is a relatively new item in seacock technology. It allows the use of an NPT threaded ball valve on the top and a NPS threaded thru-hull then threads into the bottom.
As far as I know these are currently only made by Groco. The benefit is that even if the valve wears out you simply thread a new one onto the flange. This avoids a very time consuming job of removing the thru-hull, thru-bolts and seacock and then having to re-install these parts and fair over the bolt holes.