FGT has it right about ". . . the piece at the bottom of the tailgate that ties the two sides together." It has holes for 7 strips or 8 strips (for 8 or 9 boards):

47-51 9 board rear cross bed still with your bed?

51-53 8 board rear cross bed sill

Do you have the rear sill (the rear piece that connects the bed-sides)?