It looks like an old dropwire or lead-in cable has been forced into the IDC connectors top left of your piccie. Although these connectors aren't designed to take that size wire it will probably be OK, except the connectors are sprung too far to take a second cable. This cable would normally be connected to screw terminals in a master socket.
The two connections bottom left are usually connected internally to the top left connectors, and should take the blue/white and white/blue wires, but there are some 16 -way boxes around that don't have an internal connection between the two rows of connectors.
If your box doesn't have the internal connections, you will have to try and terminate the internal cable on top of the dropwire, if the terminals will still accept it.
There is another version of the same box with both IDC and screw terminals made for this job.