These are the surface mount 50 pin 0.5mm Board to Board Double Slot Female Connectors found on the Alchitry FPGA boards. These headers allow for Alchitry FPGA Boards to stack on one another by being soldered to the topside of the board.
These connectors are sold in single units. Four of these connectors are needed for the topside of an Alchitry Board.
Documents: