3 bar (43.5 PSI) 3/4 inch BSP female x female pressure relief valve. This size is quite unusual, normally these are found in 1/2 inch size. Colour of plastic top may vary from picture (makes no difference to operation of valve).
The outlet from a pressure relief valve should be positioned in a way so as to not scald people or animals with escaping hot water or steam.
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.