Early Mustangs had a clutch adjustment at the top of the bar (to 6-20-64 ?), later Z-bars had the adjustment at the pressure plate arm. I'm not sure about the adjustment changes. A different bar was used in 66. On February 14, 66, an additional spring was used at the top of the bar to the firewall. That makes four bars for standard 260/289 engines, and four for HP engines.
Jim