When you need to hold two pairs (4 units) of TRED’s, these Extension Pins are designed to suit the TRED Pro Mounting Bracket and allow two pairs of TRED Pro, TRED HD, TRED GT, TRED 1100 or TRED 800 units to mount to them. Extension Pin set will only suit the TRED Pro Mounting Bracket, (not suitable for the TRED 1100 Mounting Bracket).