Interesting question and one I've net heard until recently. Of course on 95% of the cars this is a detail that is fairly hidden from view so it will not show up on most pictures taken of these cars. Then add to it the fairly large number of radiator supports that have been replaced over the years and the idea that its likely IMHO that someone adding a radiator support would not add the holes since they would not be seen - just a bit lazy or a who cares if you can't see it sort of mind set
Can offer that its not an early- verses late thing
Earliest/lowest VIN with holes was in the low 100's
Latest/highest VIN with holes was in the 3000's
Of the pictures of bare radiator supports on 69 and 70 Shelbys almost all of them had the holes. Of the ones that did not some of those are the result of suspect since the supports had been replaced. Of the remain couple so far they are falling into a similar production range. Not sure if this was due to a production problem, a short period production change or just mistakes possibly made by the same worker for a short period of time before it was caught and corrected.
Let me assemble some examples and the results and report back
Will look to see how many buck tags I have also. Only found one in this first group of cars I looked through and it did not have a HP stamped in it - early car.