This fitting is commonly used to plumb Scandvik pull-out sink sprayers and recessed shower kits.
It converts British Standard Pipe Parallel (straight) threads to National Pipe Straight threads.
Although it has straight threads, this nylon fitting will make a seal with a 1/2" NPT female pipe fitting in your vessel's water supply. Then you can attach the 3/8" BSP female fitting on the supply end of the shower hose.
Even though they both have 1/2" NPS threads, this fitting cannot be used with the SCA 10026 or 10027 hose adaptors.