These are standard 20mm x 20mm x 17mm (with a 4mm wide T-slot tab) it's used to connect (2) V-Slot or 20 Series T-slot aluminum extrusions together for a 90 degree connection.
Select Color. Available in Silver and Black Paint Coat
NOTE: Since the cast comer brackets have stand off teeth (for locking in the slot) in one direction you use 10mm Low Profile Screws and for the flush mount direction you use the 8mm low profile screws along with the M5 Tee Nuts. You can also sand away the stand off teeth and use 8mm on both sides.