My car has chrome cap bolts both on the face and on the under side. The bumper has square holes in both locations to hold the bolts.
See the picture below and notice the face bolts (circled in blue) have a taller profile head than the underside bolts (circled in red). Also notice the difference in the bolts in the second picture. You can see the bolt circled in red (the underside bolt) has a shorter square locking portion and the profile of the head is shorter. The face bolts will not work for the underside as the taller square portion prevents the nut from being able to be tightened. My car has the chrome bolts shown on one side but has standard hex bolts on the other. I was hoping to get the chrome bolts for both sides but I can't find anything except the ones for the face.