So, the first photo is not your car? If yours came with an OSAC valve it would have three holes there in the firewall. Two for the mounting screws, and one big one for the back side of the valve. Maybe something else was mounted there at one time.....electronic control module maybe? In '74 they decided to install the OSAC in the air cleaner housing.
No, the car in picture is not my car. The photo was taken at the Mopar Nationals years ago. I just wondered what the holes were for on my firewall. I believe this car was a 72 as well.