Thanks again, Jim. Im reconditioning a solid piece of British Sound engineering a 12-8-2 mk 2 MTR recording desk. As far as I found out many of these desks were modified right away to decrease grounding hum and I think this connector was a result of that (Ive seen a few desks with different methods of power and connection). Whoever did the modification moved the internal transformer outside the unit (still ac240v to 15v), put it in an external box, modified some of the internal wiring (just re-routed switches and fuses) and used one of the connectors I asked about. However I cant see what the've done that could have taken the grounding hum away all the electronics/ components look pretty standard according to the MTR wiring diagrams. Unless they just wanted to make the desk more portable by adding this externally connected power supply otherwise the unit would have to be carried around with a flying lead.
Eventually Ill replace the crackling sliders and have supply inside the unit without the hum of course. Firstly though Im going to take the unit back to its original wiring to see if it actually had any ground hum and work from there. Any suggestions, tips, pointers most welcome.