What "makes" a 66 Barracuda a Formula S ??

there have been several on ebay that cant be sold for 30k in really good shape. I paid 20k for the avatar car to the left. It is a non-formula s but it is a performance package car which is a C 5 car rather than a C2 formula s. I have the sway bar, commando 273, 4 gear, factory tach, limited slip and heavy-duty suspension. It is a joy to drive and gets thumbs up everywhere I go. One of the neighborhood kids had his prom pictures taken with it, him and his date. These cars are undervalued and a great choice for a nice A body on a small budget