3/4 inch x 1/2 inch BSP malleable black iron reducing hexagon nipple.
Malleable black iron plumbing fittings are used for gas / air / oil /sprinkler / heating applications. They are NOT used for fresh water.
BSP (British Standard Pipe Thread) sizes don’t match up with what you would measure using a ruler. Have a look at the BSPT link at the bottom of this page for more information about how plumbing threads are measured.