The USB socket - it's fitted so the edge of the PCB sits between the 2 rows of pins, allowing them to be soldered to each side of the board. The 3-pin row is on the top side:
Solder the pins on both sides of the board and bridge the socket case to the board as shown - be careful the solder doesn't end up in the socket though.
Bridge these pads with solder or component wire offcuts:
