For the spring I'm kind of wondering if it may be a Van Nuys thing. If it were a late 70 thing, it disappeared before 71 as its not in the 1971 AIM either. Most of the ones I've seen it on are west coast Camaro's. Did the Van Nuys plant also assemble Vega's? Vega's also have this spring. Photo's are of Mark M's 1970 Z28 restored by Muscle Restoration and Design.