Clean up the mating surfaces of the boss, plate and lock washer. Sit it on the bench, place the blade plate on the boss. Put the lock washer on and see if the tangs of the boss protrude past the lock washer. If so, the nut will tighten on the boss before it locks in , and make the plate spin. You can lightly linish the tangs down until they are just under the washer.
But Don't take too much off. If it necessary to do so then the plate will be too worn/ thin and be dangerous to fit, as I suspect it is and should really be replaced
The other washer should not need to be used as stated previously.
Fit a new lock nut and tension it up.
