I have since learned that as you get into '66 production (at least on the San Jose cars) that you will find that the rear valances have the round, rather than oval holes. As a side note, it usually seems that the valances with the smaller round holes use a smaller head Phillips pan head screw, although I have seen a mix of screws on the same car. It's interesting to note that those smaller head screws would not work well with the larger oval holes.
The above installation also used two different style screws. Note the larger well liner screws behind the bumper guards.
I've also had some earlier NOS service rear valances that have had the oval holes.
Also I have noted that sometimes, none of the screws had any body color paint.
Another thing, on most rear valances with the oval holes, since the screw heads are larger, you cannot tell which type of hole it is when it is installed.