That lower shield covers the crossover pipe on the pre-88 V6's. The 88's had the honeycomb shield wrapped around the crossover pipe and the pipe was moved outward to the left. If there was a difference in the 88 coil shield and the earlier shields, it may be that someone has replaced yours with an earlier one. But I would suspect Pontiac simply continued with the old design rather than retool to eliminate the screw hole.