EXP, in the case of the 1968 Ford Mustang you see above, stands for Experimental. You see, this is one of the cars used by the Shelby American team in the development of a long list of classic Shelby Mustang models.
The car, powered by a 390-cubic-inch V8 engine and known internally as The Green Hornet, underwent a huge number of upgrade sessions that saw it receive experimental fuel injection, independent rear suspension and unique disc brakes. That’s right, a Ford Mustang with IRS!
In a somewhat baffling turn of events, despite a top bid of $1.8 million, The Green Hornet’s reserve price wasn’t met, meaning it stays with its current owner for at least one more auction cycle.
SOURCE: http://www.autoblog.com/2013/01/19/1968-shelby-exp-500-fails-to-sell-at-barrett-jackson-despite-1/