do you have the old spec original 1 1/8 6 spline short stub pto on the tractor, and is your mower the new 1 3/8 6 spline?
if so get a 1 1/8 female to 1 3/8 male ORC, pin on type. it upsizes the pto and keeps you safe by disconnecting the mower inertia from the drivetrain in case you need to stop fast..