Bob,
Only a few early '70 GTO's were fitted with "Vacuum Operated Exhaust" (VOE), before GM put the "kabash" to it. The system used a "tiger button", which resembled a RAM AIR pull knob. When pulled, vacuum went to a solenoid which moved a baffle inside the muffler. The baffle directed exhaust through a lower restriction area of the muffler, which also enhanced the sound. To see the neat application for real, click the link below to watch the 1970 GTO commercial titled "The Humbler", which was also pulled by GM. The VOE and the commercial were too suggestive of racing, so GM dropped them.
The Humbler
Rita
