I can see another alternative that seems to easily fit the facts. While it is possible either side could be either mistaken or lying, we are talking about paint on a rock since before 1983.
It seems obvious that anything readable in 2008 would not be the original paint, whenever that might have been placed (1900? 1940? 1962?). That means someone - presumably an owner of the property or someone that was a self-appointed painter/grafiti-est - repainted it. If they did so to 'refresh' the letters, then what would be the difference (from the letterer's viewpoint) if they simply wrote it on the now blank area, instead?
As a hunting lease, Perry or anyone from his family might be on the property in the fall once or twice, or might not. There would be no way - and no particular reason - to police the area the other 9+ months of the year. So the rock may have been painted over and repainted any number of times.