One clue might be that if the grille has never been repainted/painted over, that on a 66 Shelby, they (Shelby American) had to take lacquer thinner and wipe down the horizontal grille bars in the center of the grille to remove the paint on the leading edges. On a non-GT 66 Mustang, the horizontal bars are bare chrome on the leading edge, except they were left blacked out behind the corral so they didn't show. On a 66 Shelby, if they hadn't cleaned the paint off, there would have been this blacked out square showing in the center of the grill. I remember reading somewhere that when Shelby received the first batch of 66 grilles to use in the carryover cars, they panicked when they saw this black square in the middle of the grilles, until someone realized it was just paint over the chrome, and wiped it off. Not conclusive it came out of a Shelby, but could be an indicator on whether or not the story is true. Hope this helps.