This fitting is commonly used to plumb Scandvik faucets that have 3/8" BSP male threaded supply shafts.

It converts British Standard Pipe Parallel (straight) threads to National Pipe Thread tapered threads.

The brass adapter screws onto the faucet supply shaft, converting it to a "standard" 1/2" male NPT fitting to connect your vessel's plumbing.
A rubber sealing washer is included for the straight threaded female end.