its a galvanised iron nipple, socket and nut. its for joining water pipes together so you can get water from one place to another so you can have a shower or take a dump.

ALong with 17 others who failed to answer the question. Cheers Robbo

Thanks peasea I knew the fittings as a galvinised long screw, a back nut and a socket but didn't know its use or cumulative name.
Thanks to the forum for answering my questions

Hi guys, the long thread is called a running thread, it would usually join on to the next pipe , which had a shorter thread called a backnut  thread. the socket is wound down the running thread onto the shorter backnut thread to join 2 pipes together. A grommet [hemp rolled as for a crox] would be placed between the socket and backnut. I used to use these a lot a lifetime ago!!! Maybe you can get cpd points for reading this, it seems perfect to get points on basically redundant technology which you will probably never use!!!!