So judging by their location right next to connectors and that there is one each side of the board and they connect to an exposed copper area running around the perimeter of the board, presumably for an electrical connection to a chassis that fastens around the board, I would say they are likely AC coupling between the chassis ground and the main ground of the circuit so they are connected as far as AC signals are concerned but they remain isolated as far as DC voltages are concerned.
So judging by their location right next to connectors and that there is one each side of the board and they connect to an exposed copper area running around the perimeter of the board, presumably for an electrical connection to a chassis that fastens around the board, I would say they are likely AC coupling between the chassis ground and the main ground of the circuit so they are connected as far as AC signals are concerned but they remain isolated as far as DC voltages are concerned.
