Magneto platform locking plate?

It's a strange thing, indeed. I've never seen a plate folded over (it hardly seems large enough) so I'm not sure what it locks. It apears to be purely an odd shaped washer for two fasteners...9090 is a 5/16" x 11/16" long screw that screws directly into the lower lug of the magneto platform. If yours is turning then it sounds as if the thread is indeed stripped. If the engine and magneto studs are tight then it only really prevents the assembly rotating. It doesn't need to exert any force...If it's not falling out, I'd be inclined to leave it unti the next time the mag is off.

... I've never worked out what it actually does, as it's not a tab washer and can't be folded over.