Male-male hydraulic reduction nipple used for some compressor heads that have metric threads.

The reduction is from ½ inch British BSPT to M16 metric.
Indispensable if you want to make a conversion to also use it as a vacuum pump as shown in this video:

Air Compressor to Vacuum Pump Conversion (youtube.com)

The nipple is very difficult to find because it uses a reduction from British piping (in inches) to the European metric standard (in mm), and those who produce it in metal are rightly overpricing for it.