I just went and checked the fender tag on my 74, and also checked the build sheet. Everything I have says that code A57 is the Challenger Rallye Package. In my case it included the following, J54 Bulge Hood, Black Grille, Fender scoops, Body side strobe stripe, HD Suspension, Front and rear sway bar. Rallye Instrument Cluster, etc. I can't say if the 73 differs from the 74, perhaps somebody with a known 73 Rallye can confirm the option code for 1973.
transman