For 1966: "Now added to the options list were the Koni shocks, the Detroit Locker differential, as well as the wood-rimmed steering wheel. More wheels were now available, the leftover cars having either the silver-painted steel wheels or the Crager mags, which both were also available on later produced cars. Standard wheels on the later cars were silver-painted 14 inch Magnum 500s, but the Hertz cars came with the chrome versions. More wheel options on later cars were also the 14 inch Shelby aluminum ten-spoke wheels and plain 14 inch silver-painted wheels. Most early cars came with hollow-letter Cobra valve covers while the later cars had solid letters and a black crinkle finish. "
http://www.mustangspecs.com/years/66shelby.shtml