This is the “500EX Key Hub”. It is a slightly different version of the 500 Key Hub. It is manufactured differently, as it is extruded. Also, there is a 1.124 in. diameter boss on each side of the hub flange, providing a location for 1.125 in. bore flat sprockets. Are you having a hard time keeping your sprocket concentric with your hub? This hub is the trick.

The bore is a 1/2 in. hole with two 1/8 in. keyways. Only one keyway is needed to drive most wheels, so the second one is extra.

Also, the keyways are consistently aligned with the mounting holes.