Rare self-locking hub nut used in the late '80s. Its locking tooth in the center often gets bent inward by improper installation or removal. Make sure it protrudes enough to fully engage the slot on the spindle. If not, it will work its way loose.
When installing or removing this nut, make sure the outer cage does NOT spin; the tooth in the center must remain in the spindle's groove.
This explains how to re-install it:
