The nut on the right, Franz (Moito), is what is called a Fuji nut in these parts. One advantage of this type of nut versus the nyloc is that Fuji nuts turn onto a stud with much less friction than a nyloc. For the bumper deco application this might not be a big deal, but with the more fragile rear deck lid studs the Fuji offer less hazard, and hold against loosening very well. There are other places in our cars that Fuji nuts were used. Perhaps some enthusiast will want to start a thread recording the results of a hunt for these nuts on original cars.