A57 was the fender tag code for the Monaco 500 option, but that was only for the Monaco line and I think it was dropped in 71?
On a challenger that same code was using in 73/74 for the Rallye package which consisted of the sport good, black grille, side stripes, suspension upgrade, instrument cluster, simulated side louvers and F70 x 14" WSW tires.
It was also used for the Charger's Rallye package but I am not sure what years