It is the splined version to make installing on newer mag drives a snap. This part is a direct replacement for the old three/four jaw style mag driver but the mag drive must be upgraded to the spline type to fit correctly. Use with #2500-0088 driver and #2500-0089 spyder. This piece only fits the mag drive and will not work on the magneto.
Coupler is drilled for a rare earth magnet so racer can use with a cam-sync ignition set-up.