To me the dates appear to be from a later time. They are lighter in color than the figures, which indicates youth. The older figures have had more time to be covered with desert varnish again. There are light colored rings around the buckshot similar in color to the numbers.
One date appears to be 12-19-1975. The other ones appear to be 9 - 1? - ?8. The area is in a oil field being drilled in the 50s to 70s. So there were people with time on their hands to be able to find the panel and deface it.