In the history of the muscle car I believe one of, if not the most underrated options was the drag pack option in 1969 and 1970
For 1968 the changes from a GT500 to a GT500KR were also very significant. However.....everyone knew as it was prominently displayed in the stripes on the side of the car. Visual impact is something that let others know what you had. Mopars had options like a shaker hood on a Challenger (on the Hemi Cuda it was standard), billboards, wings and louvers. On a Chevy, cowl induction hoods, racing stripes, front and rear Camaro spoilers.
On 428 ci 69-70 Mustangs and Shelbys there was nothing externally to let you know if it was a CJ or SCJ unless you count the oil cooler behind the grille, but that's a stretch.
The differences of these engines and supporting bits and pieces are getting very difficult to find....
deeper groove pulleys
gear reducers on speedo
oil cooler and related hardware
lemans rods
external dampening
etc
They are complete sleepers and especially considering the price in period.....it was practically free.
Great option and now I know why guys like Ed gravitate towards it.