A male-male hydraulic reducing pipe thread joint used for some compressor heads with metric threads.

The reduced size is from a 1/2-inch British BSPT thread to an M16 metric thread.
If you want to convert it into a vacuum pump, this connector is essential. You can refer to this video for more information:

Converting an air compressor into a vacuum pump

This pipe thread joint is hard to find because it has been reduced from the British standard