PABX master version is usually for use with PBX's-essentially the same as a BT master box but without the surge arrestor.
Can be used for extension wiring on a standard BT telephone line
when there is only one pair available. A tool to install the cabling to the IDC block can be found HERE.
BT NTE5 type master socket (most common type of BT socket-3 piece design fitted to most modern homes) also available-see HERE.