that is not a nut. that is the receiver washer. first the prop goes on. then that washer, that fits over the splines of the output shaft, then the lock washer with the tabs. then the 1 1/16th nut goes on and tightens with the proper sized socket. 50lbs torque. then bend the tabs down into the recesses of that castle looking washer. prop tool made for this and is cheap, but you can do it with regular hand tools.
seems like the nut is gone from what you've said.
seems like the nut is gone from what you've said.