Outside bib-tap wall mounting plate, suits 15mm copper and plastic plastic pipe.
Sometimes called a "wing-back elbow" or "back-plate elbow".
Chromed on brass - not mirror chrome.
1/2 inch BSP female thread.
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.