The prop nut holds the prop on. However, those little plastic nubs are guides, I think. If you look inside your spinner, there are no receptacles for those knobs. I believe that as you twist the prop on, you will be looking to have each blade going between the plastic nub and the screw receptacle. Then, when you put the spinner on, after the screw shafts are lined up, make sure the blades do not touch the sides of the spinner. There will be hardly any clearance, but there should be some. If the blades touch the spinner, loosen the prop nut and rotate the prop as needed to make sure the blades don't touch. When set, tighten everything up.

Hope that helps.