This is 6.7MM McNamum Wheel Coupling with M2.5*30 screw.

This 6.7mm ABS Hex Coupling for 60mm and 80mm Plastic mecanum wheels is a universal ABS pillar mount coupling that allows you to mount the robot wheels and mechanisms to the motor shaft. The coupling is sold separately and each hub includes a screw for fixing the motor shaft to the hub. To use a common hub, all this requires tightening screws It is compatible with 60mm and 80mm plastic mecanum Wheel. 

Package Includes:

• 1 x 6.7mm ABS HEX coupling for Mecanum Wheel
• 1 x  M2.5 x 30mm Screw