These M8 brass T-nuts are freshwater and salt water approved.  They are milled brass for superior strength and will fit any standard US track box.  The brass prevents corrosion in the connection system.  This 10 pack is a great way to keep extra T-nuts around.

Measurements:
15mm width
19mm length
8.5mm height
8.6mm width of center section with threads
3.95mm height from bottom of step