THIS PART WAS CHANGED IN 1984 from 1.345 diameter TO A LARGER DIAMETER OF 1.364 this is the smaller version.

YOU CAN TELL BY LOOKING IN THE CRANKCASE. IF THE CASE IS MACHINED INSIDE WHERE THE BACK COVER FITS IT IS PN 21-6102. IF IT IS AN AS CAST FINISH IT IS PN 21-6101