Stainless 316 Bspp Hex Nipple

That's what you've got. One end is G 3/8-19 male BSPP and the other is G 1/4-19 male BSPP, so it steps down between those two sizes in a straight configuration. Both threads are parallel BSPP, not tapered, so keep that in mind when fitting it into your system. The hex measures 22mm across the flats, which gives you a solid wrench grip during installation.

For stepping between two BSPP female ports, this fitting gives you a G 3/8-19 male BSPP on one end and a G 1/4-19 male BSPP on the other, in a straight hex nipple shape.