I just looked at a '68 st louis automatic equipped. The spray appears to been applied by shooting upwards as the various bends in metal facing upward lack black-out paint. There is paint on the engine side of the radiator flange and there is no paint on wiring grommets, bolt threads, wiring or cooler/lines that are mounted to the grille side. I did not look to see if the front valence was fitted can check if needed.
Perhaps the treatment was applied from the grille opening with an open hood?