You have to be cautious looking at these cars today...as many have had quite a bit of tin-pounding over the years.
Here's the chronology
427 Comp cars had no flares and the 'standard' 427 rear profile
When they began the street car run, it was felt that the street tires were getting lost in the rear hips, so they tried a narrower rear fender for a run of about 25 cars, somewhere in between CSX3123 thru CSX3158. They went back to the normal rear profile for the rest of the run.
The S/C variants had their rear fenders pounded out into an approx. 1 inch flare, as a pebble-catcher.
That's it... anything else was "aftermarket"
