3 bar 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 safety valve should be positioned in a way so as to not scald people or animals with escaping hot water or steam.
** BSP thread sizes are measured a special way - have a look at our information page for more details **