It's not a stretch to guess that:
A) The rivets were added to the flashgun during the original construction of the prop, and then the grips were laid on either side, just overlapping them. Sticking the grips over the rivets to hide them would not have been feasible, unless they reamed out the undersides of the grips to accommodate the rivets, so that the grips would fit flush to the tube.
...or...
B) After it was determined during filming that the d-ring needed reinforcement, the three grips closest to the rivet-end of the d-ring were stripped off, the side rivets added, and then the grips reinstalled.
A) The rivets were added to the flashgun during the original construction of the prop, and then the grips were laid on either side, just overlapping them. Sticking the grips over the rivets to hide them would not have been feasible, unless they reamed out the undersides of the grips to accommodate the rivets, so that the grips would fit flush to the tube.
...or...
B) After it was determined during filming that the d-ring needed reinforcement, the three grips closest to the rivet-end of the d-ring were stripped off, the side rivets added, and then the grips reinstalled.