Some have threaded nuts and some have the press on style washer with the teeth that grab.
I've seen both styles, believe it or not the two slightly vary in shape.
One that uses the press on washer is a little fatter and more rounded on the top and the one that uses a nut is flatter and has a straigher edges.
There could have been a mold change at some point due to a broken mold or a vendor change.
I know my old 86 Salon and my brothers 87 Salon have them missing and a friend of mine with an 88 Supreme is missing his as well and this could be due to that press on washer corroding and crumbling away.
The fenders from my 84 Calais and an 84 Diesel I use to own had the nut style so the change over could have been in 85 or 86 :?:
As for removing them the easiest way is to remove the fender, or, I haven't tried this, possibly remove the hood to get better access at the cowl area.
I tried removing them from my 84 Calais and it was hopeless without damaging anything or taking something apart.