Is it the case that if your 66 GT Mustang Coupe has the hole stamped in the firewall with the correct grommet fitted for A/C, then the car was fitted with A/C from the factory? Did they only stamp the cutout for A/C fitted cars? When I bought it had no air conditioning fitted but the previous owner had blocked the holes in the A/C grommet that was fitted to the firewall. As far as I can tell there is no such 'blank' grommet available for vehicles without A/C.